drm: Add SCDC helpers

SCDC is a mechanism defined in the HDMI 2.0 specification that allows
the source and sink devices to communicate.

This commit introduces helpers to access the SCDC and provides the
symbolic names for the various registers defined in the specification.

V2: Rebase.
V3: Added R-B from Jose.
V4: Rebase
V5: Addressed review comments from Ville
 - Handle the I2c return values in a better way (dp_dual_mode)
 - Make the macros for SCDC Major/Minor more readable, by adding
   a 'GET' in the macro names
V6: Rebase
V7: Rebase
V8: Rebase
V9: Rebase
V10: Rebase

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1489404244-16608-2-git-send-email-shashank.sharma@intel.com