drm/amdgpu: use crtc directly in drm_crtc_vblank_put()

We don't need to use &amdgpu_crtc->base there as crtc is available
in the function.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465308482-15104-9-git-send-email-gustavo@padovan.org