drm/radeon: Move radeon_ttm{init, fini} to shared location

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/radeon_ttm.c: At top level:
 drivers/gpu/drm/radeon/radeon_ttm.c:817:5: warning: no previous prototype for ‘radeon_ttm_init’ [-Wmissing-prototypes]
 817 | int radeon_ttm_init(struct radeon_device *rdev)
 | ^~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_ttm.c:878:6: warning: no previous prototype for ‘radeon_ttm_fini’ [-Wmissing-prototypes]
 878 | void radeon_ttm_fini(struct radeon_device *rdev)
 | ^~~~~~~~~~~~~~~

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>