drm/i915/dp: Disable DSC in source by disabling DSS CTL bits

drm/i915/dp: Disable DSC in source by disabling DSS CTL bits

1. Disable Left/right VDSC branch in DSS Ctrl reg
    depending on the number of VDSC engines being used
2. Disable joiner in DSS Ctrl reg

v4:
* Remove encoder, make crtc_state const (Ville)
v3 (From Manasi):
* Add Disable PG2 for VDSC on eDP
v2 (From Manasi):
* Use old_crtc_state to find dsc params
* Add a condition to disable only if
dsc state compression is enabled
* Use correct DSS CTL regs

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181128202628.20238-12-manasi.d.navare@intel.com