drm/amd/display: Fix assert being hit with GAMCOR memory shut down

[Why]
A call to BREAK_TO_DEBUGGER is triggered when powering down memory

[How]
Remove the check for GAMCOR_MEM_PWR_STATE when powering off the memory

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Jacky Liao <ziyu.liao@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>