drm/amdkfd: Track GPU memory utilization per process

Track GPU VRAM usage on a per process basis and report it through
sysfs.

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>