drm/amd/display: runtime select dmub emulatior.

[Why & How]
Add emul specific hw function to dmub, in case of
emulator is created, we can runtime switch between
dmub emulator or dmub uC via is_virtual flag in dmub.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>