drm/xe/tests: Change TEST_VRAM to work with 32-bit resource_size_t

drm/xe/tests: Change TEST_VRAM to work with 32-bit resource_size_t

We've picked the value of TEST_VRAM to match real VRAM size as found
on the machines used by the CI, but that didn't work well on kernels
that have 32-bit resource_size_t. Use smaller value instead.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/intel-xe/20260227011639.GA1683727@ax162/
Fixes: cbe29da6f7c0 ("drm/xe/tests: Add KUnit tests for new VRAM fair provisioning")
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://patch.msgid.link/20260227160010.12425-1-michal.wajdeczko@intel.com