drm/i915: Drop NULL fb check from intel_fb_uses_dpt()

intel_fb_uses_dpt() should not be called with a NULL fb, so
drop the check.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231207193441.20206-6-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>