drm/amdkfd: label internally used symbols as static

Used sparse(make C=1) to find these loose ends.

v2:
removed unwanted extra line

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>