drm/i915: drop crtc checking from assert_shared_dpll

The hw state readout code for the pipe config will now check
this for us, so rip out this hand-rolled complexity.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>