drm/i915: Store active_pipes bitmask in cdclk state

Let's add a copy of the active_pipes bitmask into the cdclk_state.
While this is duplicating a bit of information we may already
have elsewhere, I think it's worth it to decopule the cdclk stuff
from whatever else wants to use that bitmask. Also we want to get
rid of all the old ad-hoc global state which is what the current
bitmask is, so this removes one obstacle.

The one extra thing we have to remember is write locking the cdclk
state whenever the bitmask changes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200120174728.21095-19-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>