drm/amdkfd: Document and fix GTT BO kmap API

Removed an unused parameter from two functions and added kernel-doc
comments.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>