drm/amd/display: Fix for outbox1 ring buffer typecasting issue

[WHY]
Compiler warning "pointer to integer of different size" reported on
outbox1 ring buffer address typecasting.

Reported-by: kernel test robot <lkp@intel.com>

[HOW]
Fixed the issue by typecasting with character pointer.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>