drm/amdgpu: fix function scopes

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

 warning: no previous prototype for 'amdgpu_vkms_output_init' [-Wmissing-prototypes]
 399 | int amdgpu_vkms_output_init(struct drm_device *dev,
     |     ^~~~~~~~~~~~~~~~~~~~~~~

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