drm/xe: Move the FORCE_WC assert into __xe_pin_fb_vma()

drm/xe: Move the FORCE_WC assert into __xe_pin_fb_vma()

No need to bother the higher level pinning code with the
FORCE_WC assert. Move it into the lower level function.

This also introduces the check to intel_fb_pin_to_ggtt(), which
(for the moment) is still used for the fbdev bo setup. But this
is all display stuff we're talking about here so having the check
is correct everywhere.

v2: Pimp the commit message a bit (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260508143426.26504-8-ville.syrjala@linux.intel.com