[Why]
If DMUB is used, LVTMA VBIOS call can be used to control eDP instead of
tranditional transmitter control.  Interface is agreed with VBIOS for
eDP to use this new path to program LVTMA registers.

[How]
Create DAL interface to send DMUB command for LVTMA as currently
implemented in VBIOS.
Signed-off-by: Chris Park <Chris.Park@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>