drm/core: Allow drivers allocate a subclass of struct drm_framebuffer

Allow allocating a specialized version of struct drm_framebuffer
by moving the actual fb allocation out of drm_gem_fb_create_with_funcs();
the respective functions names are adjusted to reflect that fact.
Please note, though, that standard size checks are performed on buffers,
so the drm_gem_fb_init_with_funcs() is useful for cases where those
standard size checks are appropriate or at least don't conflict the
checks to be performed in the specialized case.

Thanks to this change the drivers can call drm_gem_fb_init_with_funcs()
having allocated their special version of struct drm_framebuffer, exactly
the way the new version of drm_gem_fb_create_with_funcs() does.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: James Qian Wang <james.qian.wang@arm.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200311145541.29186-2-andrzej.p@collabora.com