drm/radeon: make functions alloc_pasid and free_pasid static

The functions alloc_pasid  and free_pasid are local to the
source and do not need to be in global scope, so make them static.

Cleans up sparse warnings:
warning: symbol 'alloc_pasid' was not declared. Should it be static?
warning: symbol 'free_pasid' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>