drm/i915: Drop pipe_enable checks in vblank funcs

With Ville's rework to use drm_crtc_vblank_on/off the core will take
care of rejecting drm_vblank_get calls when the pipe is off. Also the
core won't call the get_vblank_counter hooks in that case either. And
since we've dropped ums support recently we can now remove these
hacks, yay!

Noticed while trying to answer questions Laurent had about how the new
atomic helpers work.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>