drm/amdkfd: fix function scopes

 This turns previously global functions into static, thus removing
 compile-time warnings such as:

 warning: no previous prototype for 'pm_set_resources_vi' [-Wmissing-prototypes]
 113 | int pm_set_resources_vi(struct packet_manager *pm, uint32_t *buffer,
     |     ^~~~~~~~~~~~~~~~~~~

Signed-off-by: Isabella Basso <isabbasso@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>