drm/xe/fbdev: Fix BIOS FB vs. stolen size check

drm/xe/fbdev: Fix BIOS FB vs. stolen size check

Looks like stolen->size is in bytes, not pages. Remove the
bogus PAGE_SHIFT stuff.

Also for some random reason xe rejects the FB if it takes up
exactly half of stolen, whereas i915 allows it to be used
in that case. Adjust xe to follow the i915 rule for consistency.

v2: rebase related updates

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Acked-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patch.msgid.link/20260220170908.201422-2-vinod.govindapillai@intel.com