drm/amd/display: Skip querying caps when DMCUB emulation is in use

[Why]
Workaround to avoid accessing DMCUB state too early if the emulator
is in use - we don't support any of the features the caps are querying
with emulation anyway.

[How]
Guard the query if emulation is in use.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>