drm/amdkfd: helper to convert gpu id and idx

svm range uses gpu bitmap to store which GPU svm range maps to.
Application pass driver gpu id to specify GPU, the helper is needed to
convert gpu id to gpu bitmap idx.

Access through kfd_process_device pointers array from kfd_process.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>