drm/i915/dsc: move crtc state dp_dsc_cfg member under dsc as config

DSC isn't DP specific, so remove the dp_ prefix from the crtc state
member name. Also moving the member under the dsc sub-struct gives us
enough context to allow shortening the name to just config. No
functional changes.

Cc: Manasi Navare <manasi.d.navare@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/20191022133414.8293-2-jani.nikula@intel.com