drm/dp: Add definitions for eDP Rev 1.4a and 1.4b

VESA eDP 1.4 specification has separate fields defined in
EDP_DPCD_REV for eDP 1.4a and 1.4b eDP revisions.
This patch defines those. Found this when one of my eDP panels
advertises eDP 1.4a (04h) in the EDP_DPCD_REV DPCD field.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181009002351.23085-1-manasi.d.navare@intel.com