drm/i915: Introduce intel_calc_active_pipes()

Extract a small helper to compute the active pipes bitmask
based on the old bitmask + the crtcs in the atomic state.
I want to decouple the cdclk state entirely from the current
global state so I want to track the active pipes also inside
the (to be introduced) full cdclk state.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200120174728.21095-17-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>