drm/i915: Relocate some other plane fb related stuff into intel_fb.c

drm/i915: Relocate some other plane fb related stuff into intel_fb.c

Move intel_fb_xy_to_linear() and intel_add_fb_offsets()
These are technially sitting somewhere between plane vs. fb code,
but we do have a bunch of code like that in intel_fb.c anyway.
Might need to think about splitting intel_fb.c into pure fb vs.
plane->fb related stuff somehow, but dunno if that's even feasible.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250213150220.13580-12-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>