drm/radeon: use dma_resv_wait_timeout() instead of manually waiting

Don't touch the exclusive fence manually here, but rather use the
general dma_resv function. We did that for better hw reset handling but
this doesn't necessary work correctly.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deuche <alexander.deucher@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211028132630.2330-6-christian.koenig@amd.com