drm/i915/bdw: Add support for DRRS to switch RR

For Broadwell, there is one instance of Transcoder MN values per transcoder.
For dynamic switching between multiple refreshr rates, M/N values may be
reprogrammed on the fly. Link N programming triggers update of all data and
link M & N registers and the new M/N values will be used in the next frame
that is output.

V2: [By Ram]: intel_dp_set_m_n() is rewritten to accommodate
	gen >= 8 [Rodrigo]
V3: Coding style correction [Ram]
V4: [By Ram] intel_dp_set_m_n modifications are moved into a
	separate patch, retaining only DRRS related changes here [Rodrigo]

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: Pradeep Bhat <pradeep.bhat@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>