drm/i915/dsc: add basic hardware state readout support

Add basic hardware state readout for DSC, and check the most relevant
details in the state checker.

v2:
- check for DSC power before reading its state
- check if source supports DSC at all

As a side effect, this should also get the power domains for the enabled
DSC on takeover, and subsequently disable DSC if it's not needed.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3fb018cf9bd9a4c275aab389b6ec0f2a4e938bb9.1575974743.git.jani.nikula@intel.com