drm/i915: Constify 'fb' in during pinning

drm/i915: Constify 'fb' in during pinning

Make the 'fb' pointers const in the pinning code. We never
want to mutate these. Also nuke a few aliasing fb vs. intel_fb
cases by just using the more specific type everywhere in the
same function.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240506125718.26001-7-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>