drm/radeon: delete useless function return values & remove meaningless if(r) check code

Function radeon_fence_driver_init always returns success,
the function type maybe coule be changed to void.
This patch first delete the check of the return
value of the function call radeon_fence_driver_init, then,
optimise the function declaration and function to void type.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>