drm/i915/dp: Enable/Disable DSC in DP Sink

This patch enables decompression support in sink device
before link training and disables the same during the
DDI disabling.

v3 (From manasi):
* Pass bool state to enable/disable (Ville)
v2:(From Manasi)
* Change the enable/disable function to take crtc_state
instead of intel_dp as an argument (Manasi)
* Use the compression_enable flag as part of crtc_state (Manasi)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Gaurav K Singh <gaurav.k.singh@intel.com>
Signed-off-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181128202628.20238-6-manasi.d.navare@intel.com