drm/vmwgfx: use drm_crtc_vblank_crtc()

drm/vmwgfx: use drm_crtc_vblank_crtc()

We have drm_crtc_vblank_crtc() to get the struct drm_vblank_crtc pointer
for a crtc. Use it instead of poking at dev->vblank[] directly.

Cc: Zack Rusin <zack.rusin@broadcom.com>
Cc: Broadcom internal kernel review list <bcm-kernel-feedback-list@broadcom.com>
Reviewed-by: Ian Forbes <ian.forbes@broadcom.com>
Link: https://patch.msgid.link/5157c2e927676aad75348855cf7b6745cba90003.1762513240.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>