drm/amdgpu/virt: don't dereference undefined 'module' struct

drm/amdgpu/virt: don't dereference undefined 'module' struct

Accessing the THIS_MODULE directly is only possible when modules
are enabled, otherwise we get a build failure:

drivers/gpu/drm/amd/amdgpu/amdgpu_virt.c: In function 'amdgpu_virt_init_data_exchange':
drivers/gpu/drm/amd/amdgpu/amdgpu_virt.c:331:20: error: dereferencing pointer to incomplete type 'struct module'

Further, THIS_MODULE is NULL when the driver is built-in, so the
code would likely cause a NULL pointer dereference.

This adds an #ifdef check to avoid the compile-time error, plus
a NULL pointer check before dereferencing THIS_MODULE. It might
be better to find a way to avoid using the module version
altogether.

Fixes: 2dc8f81e4f82 ("drm/amdgpu: SR-IOV data exchange between PF&VF")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-By: Xiangliang Yu <Xiangliang.Yu@amd.com>