drm/i915/gtt: flush the scratch page

The scratch page is directly visible in the users address space, and
while this is forced as CACHE_LLC, by the kernel, we still have to
contend with things like "Bypass-LLC" MOCS. So just flush no matter
what.

v2(Thomas):
  - Make sure we use drm_clflush_virt_range here, in case clflush support
    is missing.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211028092638.3142258-1-matthew.auld@intel.com