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

drivers/gpu/drm/i915/i915_drv.h

@@ -3342,6 +3342,7 @@ extern bool intel_set_memory_cxsr(struct drm_i915_private *dev_priv,
 				  bool enable);
 void intel_dsc_enable(struct intel_encoder *encoder,
 		      const struct intel_crtc_state *crtc_state);
+void intel_dsc_disable(const struct intel_crtc_state *crtc_state);
 
 int i915_reg_read_ioctl(struct drm_device *dev, void *data,
 			struct drm_file *file);