drm/amdgpu: Clear the hotplug interrupt ack bit before hpd initialization

Problem:
The computer in the bios initialization process, unplug the HDMI display,
wait until the system up, plug in the HDMI display, did not enter the
hotplug interrupt function, the display is not bright.

Fix:
After the above problem occurs, and the hpd ack interrupt bit is 1,
the interrupt should be cleared during hpd_init initialization so that
when the driver is ready, it can respond to the hpd interrupt normally.

Signed-off-by: Qiang Ma <maqianga@uniontech.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>