drm/i915: Convert to CRTC VBLANK callbacks

VBLANK callbacks in struct drm_driver are deprecated in favor of their
equivalents in struct drm_crtc_funcs. Convert i915 over.

The callback struct drm_driver.get_scanout_position() is deprecated
in favor of struct drm_crtc_helper_funcs.get_scanout_position().
i915 doesn't use CRTC helpers. Instead pass i915's implementation of
get_scanout_position() to DRM core's
drm_crtc_vblank_helper_get_vblank_timestamp_internal().

v3:
	* rename dcrtc to _crtc
	* use intel_ prefix for i915_crtc_get_vblank_timestamp()
	* update for drm_crtc_vblank_helper_get_vblank_timestamp_internal()
v2:
	* use DRM's implementation of get_vblank_timestamp()
	* simplify function names

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123135943.24140-8-tzimmermann@suse.de