drm/i915/fbdev: make intel_framebuffer_create() error return handling explicit

drm/i915/fbdev: make intel_framebuffer_create() error return handling explicit

It's sketchy to pass error pointers via to_intel_framebuffer(). It
probably works as long as struct intel_framebuffer embeds struct
drm_framebuffer at offset 0, but be explicit about it.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/17631db227d527d6c67f5d6b67adec1ff8dc6f8d.1758184771.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>