drm/i915/frontbuffer: convert frontbuffer->obj to struct drm_gem_object

drm/i915/frontbuffer: convert frontbuffer->obj to struct drm_gem_object

Prefer the driver agnostic struct drm_gem_object over i915 specific
struct drm_i915_gem_object.

Do some opportunistic struct intel_display conversions while at it,
because it's more convenient to deal with.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/51bdb3c9b798e28bd70c259fc3874d80bc9b7443.1726589119.git.jani.nikula@intel.com