drm/i915/sseu: fix max_subslices array-index-out-of-bounds access

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

drm/i915/sseu: fix max_subslices array-index-out-of-bounds access

It seems that commit bc3c5e0809ae ("drm/i915/sseu: Don't try to store EU
mask internally in UAPI format") exposed a potential out-of-bounds
access, reported by UBSAN as following on a laptop with a gen 11 i915
card:

UBSAN: array-index-out-of-bounds in drivers/gpu/drm/i915/gt/intel_sseu.c:65:27
index 6 is out of range for type 'u16 [6]'
CPU: 2 PID: 165 Comm: systemd-udevd Not tainted 6.2.0-9-generic #9-Ubuntu
Hardware name: Dell Inc. XPS 13 9300/077Y9N, BIOS 1.11.0 03/22/2022
Call Trace:
<TASK>
show_stack+0x4e/0x61
dump_stack_lvl+0x4a/0x6f
dump_stack+0x10/0x18
ubsan_epilogue+0x9/0x3a
__ubsan_handle_out_of_bounds.cold+0x42/0x47
gen11_compute_sseu_info+0x121/0x130 [i915]
intel_sseu_info_init+0x15d/0x2b0 [i915]
intel_gt_init_mmio+0x23/0x40 [i915]
i915_driver_mmio_probe+0x129/0x400 [i915]
? intel_gt_probe_all+0x91/0x2e0 [i915]
i915_driver_probe+0xe1/0x3f0 [i915]
? drm_privacy_screen_get+0x16d/0x190 [drm]
? acpi_dev_found+0x64/0x80
i915_pci_probe+0xac/0x1b0 [i915]
...

According to the definition of sseu_dev_info, eu_mask->hsw is limited to
a maximum of GEN_MAX_SS_PER_HSW_SLICE (6) sub-slices, but
gen11_sseu_info_init() can potentially set 8 sub-slices, in the
!IS_JSL_EHL(gt->i915) case.

Fix this by reserving up to 8 slots for max_subslices in the eu_mask
struct.

(cherry picked from commit 3cba09a6ac86ea1d456909626eb2685596c07822)