drm/i915/vblank: use drm_crtc_vblank_crtc() instead of open-coding

There's a helper for drm->vblank[drm_crtc_index(crtc)], use it.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/78198ade92da1d978f2032c50b3b50e3f8f01701.1724342644.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>