drm/amdgpu: Fix buffer size in gfx_v9_4_3_init_ cp_compute_microcode() and rlc_microcode()

In the Linux kernel, the following vulnerability has been resolved:

drm/amdgpu: Fix buffer size in gfx_v9_4_3_init_ cp_compute_microcode() and rlc_microcode()

The function gfx_v9_4_3_init_microcode in gfx_v9_4_3.c was generating
about potential truncation of output when using the snprintf function.
The issue was due to the size of the buffer 'ucode_prefix' being too
small to accommodate the maximum possible length of the string being
written into it.

The string being written is "amdgpu/%s_mec.bin" or "amdgpu/%s_rlc.bin",
where %s is replaced by the value of 'chip_name'. The length of this
string without the %s is 16 characters. The warning message indicated
that 'chip_name' could be up to 29 characters long, resulting in a total
of 45 characters, which exceeds the buffer size of 30 characters.

To resolve this issue, the size of the 'ucode_prefix' buffer has been
reduced from 30 to 15. This ensures that the maximum possible length of
the string being written into the buffer will not exceed its size, thus
preventing potential buffer overflow and truncation issues.

Fixes the below with gcc W=1:
drivers/gpu/drm/amd/amdgpu/gfx_v9_4_3.c: In function ‘gfx_v9_4_3_early_init’:
drivers/gpu/drm/amd/amdgpu/gfx_v9_4_3.c:379:52: warning: ‘%s’ directive output may be truncated writing up to 29 bytes into a region of size 23 [-Wformat-truncation=]
379 | snprintf(fw_name, sizeof(fw_name), "amdgpu/%s_rlc.bin", chip_name);
| ^~
......
439 | r = gfx_v9_4_3_init_rlc_microcode(adev, ucode_prefix);
| ~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/gfx_v9_4_3.c:379:9: note: ‘snprintf’ output between 16 and 45 bytes into a destination of size 30
379 | snprintf(fw_name, sizeof(fw_name), "amdgpu/%s_rlc.bin", chip_name);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/gfx_v9_4_3.c:413:52: warning: ‘%s’ directive output may be truncated writing up to 29 bytes into a region of size 23 [-Wformat-truncation=]
413 | snprintf(fw_name, sizeof(fw_name), "amdgpu/%s_mec.bin", chip_name);
| ^~
......
443 | r = gfx_v9_4_3_init_cp_compute_microcode(adev, ucode_prefix);
| ~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/gfx_v9_4_3.c:413:9: note: ‘snprintf’ output between 16 and 45 bytes into a destination of size 30
413 | snprintf(fw_name, sizeof(fw_name), "amdgpu/%s_mec.bin", chip_name);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~