drm/amd/display: Remove assert from PHY state update

[Why & How]
In some cases, there are calls to transition from TX_ON to TX_ON. This is
expected, so do not assert. However, these are redundant, so return
prematurely.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Taimur Hassan <Syed.Hassan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>