drm/i915/display: use intel_bo_to_drm_bo in intel_fb.c

We are preparing for Xe driver. I915 and Xe object implementation are
differing. Do not use  i915_gem_object->base directly. Instead use
intel_bo_to_drm_bo.

Also use drm_gem_object_put instead of i915_gem_object_put. This should be
ok as i915_gem_object_put is really just doing 	__drm_gem_object_put.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231203114840.841311-2-jouni.hogander@intel.com