VBLANK callbacks in struct drm_driver are deprecated in favor of
their equivalents in struct drm_crtc_funcs. Convert amdgpu over.

v2:
	* don't wrap existing functions; change signature instead
Signed-off-by: NThomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123135943.24140-6-tzimmermann@suse.de