drm/panthor: harden firmware build-info bounds checks

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

drm/panthor: harden firmware build-info bounds checks

panthor_fw_read_build_info() checks whether the metadata range fits in the
firmware image with hdr.meta_start + hdr.meta_size. Both fields are u32, so
the addition can wrap and let an out-of-bounds range pass validation.

The function also reads the "git_sha: " prefix without first checking that
the metadata is long enough, and meta_size == 0 can underflow the NULL
terminator index.

Use subtraction-based bounds checking and reject metadata that is too short
to contain the expected prefix and trailing NULL byte.