drm/{i915, xe}/fbdev: add intel_fbdev_fb_bo_destroy()

drm/{i915, xe}/fbdev: add intel_fbdev_fb_bo_destroy()

i915 and xe do different things on the failure path; i915 calls
drm_gem_object_put() while xe calls xe_bo_unpin_map_no_vm(). Add a
helper to enable further refactoring.

v2: Call drm_gem_object_put() on intel_fbdev_fb_bo_destroy()

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/22bc3c3158f5a22ab258ada8684766fdf75fefec.1758184771.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>