drm: Add LT-tunable PHY repeater mode operations

LT-tunable PHY Repeaters can operate in two different modes: transparent
(default) and non-transparent. The value 0x55 specifies the transparent
mode, and 0xaa represents the non-transparent; this commit adds these
two values as definitions.

Cc: Abdoulaye Berthe <Abdoulaye.Berthe@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Abdoulaye Berthe <Abdoulaye.Berthe@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015134010.26zwopwnrbsmz5az@outlook.office365.com