drm/amdgpu: Make struct amdgpu_atif private to amdgpu_acpi.c

Currently, there is nothing in amdgpu that actually uses these structs
other than amdgpu_acpi.c. Additionally, since we're about to start
saving the correct ACPI handle to use for calling ATIF in this struct
this saves us from having to handle making sure that the acpi_handle
(and by proxy, the type definition for acpi_handle and all of the other
acpi headers) doesn't need to be included within the amdgpu_drv struct
itself. This follows the example set by amdgpu_atpx_handler.c.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>