drm/i915: Use sseu size for determining eu_regs[]

eu_regs[] is written 2*max_slices times (like s_reg[]) but oddly read
2*max_slices + max_subslices/2 times. Allocate the array large enough
for the writes to avoid overwriting our stack and worry about the logic
later.

Fixes: 7aa0b14e ("drm/i915: Remove variable length arrays from sseu debugfs printers")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105479


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180313113149.1094-1-chris@chris-wilson.co.uk