drm/i915/fb: convert intel_framebuffer_create() to struct drm_gem_object

drm/i915/fb: convert intel_framebuffer_create() to struct drm_gem_object

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

Move the declaration to the right place while at it.

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/5120aa0f5e49e95526b3ac20c1325bac1d95aa21.1726589119.git.jani.nikula@intel.com