drm/i915/dp: Add compute routine for DP VSC SDP

In order to support state readout for DP VSC SDP, we need to have a
structure which holds DP VSC SDP payload data such as
"union hdmi_infoframe drm" which is used for DRM infoframe.
It adds a struct drm_dp_vsc_sdp vsc to intel_crtc_state.infoframes.

And it stores computed dp vsc sdp to infoframes.vsc of crtc state.
While computing we'll also fill out the inforames.enable bitmask
appropriately.

The compute routine follows DP 1.4 spec [Table 2-117: VSC SDP Payload for
DB16 through DB18].

v3: Replace a structure name to drm_dp_vsc_sdp from intel_dp_vsc_sdp
v5:
  - Rebased
  - Add warning where a bpc is 6 and a pixel format is RGB.
v7: Fix the wrong check of combination bpc 6 and RGB pixelformat

Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211074657.231405-3-gwan-gyeong.mun@intel.com