drm: Add DP1.4 VSC SDP Payload related Data Structures

It adds new enumeration definitions for VSC SDP Payload for Pixel
Encoding/Colorimetry Format.
And it adds a new drm data structure for DP VSC SDP.

enum dp_colorspace and enum dp_colorimetry correspond "Pixel Encoding and
Colorimetry Formats". enum dp_dynamic_range corresponds "Dynamic Range".
And enum dp_content_type corresponds "Content Type"
All of them are based on DP 1.4 spec [Table 2-117: VSC SDP Payload for
DB16 through DB18].

v3: Add a new drm data structure for DP VSC SDP
v5: Addressed review comments from Uma
    - Add kernel docs for added data structures
    - Rename enum dp_colorspace to dp_pixelformat
    - Polish commit message
    - Fix typos
    - Drop self-explanatory comments

Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211074657.231405-2-gwan-gyeong.mun@intel.com