drm/amdgpu: fix location of prototype for amdgpu_kms_compat_ioctl

This fixes the warning below by changing the prototype to a location
that's actually included by the .c files that call
amdgpu_kms_compat_ioctl:

 warning: no previous prototype for ‘amdgpu_kms_compat_ioctl’
 [-Wmissing-prototypes]
 37 | long amdgpu_kms_compat_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
    |      ^~~~~~~~~~~~~~~~~~~~~~~

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