drm/amdgpu: perform srbm soft reset always on SDMA resume

This can address the random SDMA hang after pci config reset
seen on Hawaii.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Tested-by: Sandeep Raghuraman <sandy.8925@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>