drm/amd/display: Only one display lights up while using MST hub

[Why]
With the addition of dc_lock acquire before dc_link_handle_hpd_rx_irq,
there will be a deadlock situation where commit state sends a request
for payload allocation on MST and wait for HPD to process DOWN_REP.

[How]
Move forward the MST message handling in handle_hpd_rx_irq so that
it will not rely on call to dc_link_handle_hpd_rx_irq.

Signed-off-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>