drm: set the type of the drm_framebuffer::fbdev field

The fbdev field of the drm_framebuffer structure is always used to store
a pointer to a fb_info, so there is no reason for it to be void*.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>