drm/i915: Add encoder->is_clock_enabled()

Support reading out the current state of the DDI clock.

Not sure we really want this. Seems a bit excessive just to
restore the debug print to icl_sanitize_encoder_pll_mapping()?
But maybe there's more use for it?

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210224144214.24803-6-ville.syrjala@linux.intel.com


Reviewed-by: Mika Kahola <mika.kahola@intel.com>