drm/amdgpu: clean function declarations in amdgpu_ttm.c up

We get 2 warnings when building kernel with W=1:

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c:985:5: warning: no previous prototype for 'amdgpu_ttm_init' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c:1092:6: warning: no previous prototype for 'amdgpu_ttm_fini' [-Wmissing-prototypes]

In fact, both functions are declared in
drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c, but should be declared in
a header file, thus can be recognized in other file.

So this patch moves the declarations into
drivers/gpu/drm/amd/amdgpu/amdgpu.h.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>