drm/amdkfd: sparse: Fix warning in reading SDMA counters

Add __user annotation to fix related sparse warning while reading
SDMA counters from userland.
Also, rework the read SDMA counters function by removing redundant
checks.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>