drm/i915: Assign a new pll from the crtc check function, v2.

It saves another loop over all crtc's in the state, and computing
clock is more of a per crtc thing.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by(IVB): Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>