drm/i915/dp: Cache the DP/eDP DSC DPCD register set on Hotplug/eDP Init

DSC is supported on eDP starting GEN 10 display (on GLK) and on DP starting
GEN 11.
This patch implements the discovery phase of DSC. On hotplug,
source reads the DSC DPCD register set (0x00060 - 0x0006F) to
read the decompression capabilities of the sink device.
This entire block of registers is cached in intel_dp so that
capability information can be used during DSC configuration
phase during compute_config phase of the modeset.
For eDP, this caching happens during the eDP initialization.
This caching is done only for eDP and DP rev >= 1.4

v5:
* Fix the block comment (Gaurav)
* Fix the commit message DSC DPCD addresses (Gaurav)
* Use DRM_ERROR for dpcd_read fail (Gaurav,Anusha)
v4:
* Cache these only for Gen >= 11
v3:
* Remove the dsc_sink_support field in intel_dp (Jani N)
v2:
* Clear the cached registers on hotplug always (Jani N)
* Combine the eDP and DP caching in same function (Jani N)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: 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>
Reviewed-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181031001923.31442-3-manasi.d.navare@intel.com

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