drm/amd/display: Add CP_IRQ clear capability

[Why]
Currently we do not clear the CP_IRQ bit upon receiving it.

[How]
Added a function to clear CP_IRQ bit.

Signed-off-by: Harmanprit Tatla <harmanprit.tatla@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>