drm/i915: Move GEM BO inside drm_framebuffer

Since drm_framebuffer can now store GEM objects directly, place them
there rather than in our own subclass.

v2: Only hold a single reference per framebuffer, not per plane. (Ville)
v3: Drop NULL check in intel_fb_obj. (Ville)

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: intel-gfx@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20180518143008.4120-2-daniels@collabora.com