drm/amd/display: map TRANSMITTER_UNIPHY_x to LINK_REGS_x

[Why]
The enum value for TRANSMITTER_UNIPHY_G is 9. In resource dc_xx_resource
file structure link_enc_regs[], the TRANSMITTER_UNIPHY_G registers are
initialized at index 6. Due to this mismatch, if monitor is attached to
port using TRANSMITTER_UNIPHY_G then the monitor blanks out.

[How]
add function map_transmitter_id_to_phy_instance() and use the function
to map enum transmitter to link regs.

Signed-off-by: Yogesh Mohan Marimuthu <yogesh.mohanmarimuthu@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>