drm/amd/display: Remove unused encoder types

drm/amd/display: Remove unused encoder types

[Why&How]
We only support ENCODER_ID_INTERNAL_UNIPHY encoders now, so NUTMEG & TRAVIS
can be removed from translate_encoder_to_transmitter.

Also refactor to use local variables of transmitter to exit early.

V2: Fix construct_phy check for  TRANSMITTER_UKNOWN

Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>