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

It seems that commit bc3c5e08 ("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.
Reported-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Fixes: bc3c5e08 ("drm/i915/sseu: Don't try to store EU mask internally in UAPI format")
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230220171858.131416-1-andrea.righi@canonical.com

drivers/gpu/drm/i915/gt/intel_sseu.h

@@ -27,7 +27,7 @@ struct drm_printer;
  * is only relevant to pre-Xe_HP platforms (Xe_HP and beyond use the
  * I915_MAX_SS_FUSE_BITS value below).
  */
-#define GEN_MAX_SS_PER_HSW_SLICE	6
+#define GEN_MAX_SS_PER_HSW_SLICE	8
 
 /*
  * Maximum number of 32-bit registers used by hardware to express the