drm/gpuvm: Fix doc comments

Correctly summerize drm_gpuvm_sm_map/unmap, and fix the parameter order
and names.  Just something I noticed in passing.

v2: Don't rename the arg names in prototypes to match function
    declarations [Danilo]

Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Tested-by: Antonino Maniscalco <antomani103@gmail.com>
Reviewed-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/661456/