drm/amdgpu:add lock_reset for SRIOV

this lock is used for sriov_gpu_reset, only get this mutex
can run into sriov_gpu_reset.

we have couple source triggers gpu_reset for SRIOV:
1) submit timedout and trigger reset voluntarily
2) invalid instruction detected by ENGINE and trigger reset voluntarily
2) hypervisor found world switch hang and trigger flr and notify guest to
   do reset.

all need take care and we need a mutex to protect the consistency of
reset routine.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>