drm/i915: Initialize drm_driver vblank funcs at compile time

Move the .get_vblank_timestamp() and .get_scanout_position()
initialization to happen at compile time. No point in delaying
it since we always assign the same functions.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190619170842.20579-5-ville.syrjala@linux.intel.com