drm: Add logging function for DP VSC SDP

When receiving video it is very useful to be able to log DP VSC SDP.
This greatly simplifies debugging.

v2: Minor style fix
v3: Move logging functions to drm core [Jani N]
v5: Rebased
v10: Rebased

Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200514060732.3378396-4-gwan-gyeong.mun@intel.com