drm/amd/display: Avoid gpio conflict on MST branch

[Why]
Similar to SST branch, gpio conflict also needs to be avoided on
MST. Without doing so, there is a chance that gpio conflict will
occur if multiple gpio interrupts arrive simultaneously.

[How]
By mutex locking/unlocking &aconnector->hpd_lock,
we won't get gpio conflict when handling hpd.

Signed-off-by: Zhan Liu <zhan.liu@amd.com>
Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Acked-by: Zhan Liu <zhan.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>