drm: Refactor framebuffer creation to allow internal use (v2)

Refactor DRM framebuffer creation into a new function that returns a
struct drm_framebuffer directly.  The upcoming universal cursor support
will want to create framebuffers internally to wrap cursor buffers, so
we want to be able to share that framebuffer creation with the
drm_mode_addfb2 ioctl handler.

v2: Take struct drm_mode_fb_cmd2 parameter directly rather than void*

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Pallavi <G&lt;pallavi.g@intel.com>
Acked-by: Dave Airlie <airlied@linux.ie>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>