drm/gma500: use drm_crtc_vblank_crtc()

drm/gma500: 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.

However, we also need to get the crtc to start with. We could use
drm_crtc_from_index(), but refactor to use drm_for_each_crtc() instead.

This is all a bit tedious, and perhaps the driver shouldn't be poking at
vblank->enabled directly in the first place. But at least hide away the
dev->vblank[] access in drm_vblank.c where it belongs.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patch.msgid.link/27b2c6772c68120d0d5ec28477db0d993743e955.1762513240.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>