drm/radeon/radeon_legacy_encoders: Move 'radeon_add_legacy_encoder's prototype to shared location

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

 drivers/gpu/drm/radeon/radeon_legacy_encoders.c:1746:1: warning: no previous prototype for ‘radeon_add_legacy_encoder’ [-Wmissing-prototypes]
 1746 | radeon_add_legacy_encoder(struct drm_device *dev, uint32_t encoder_enum, uint32_t supported_device)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~

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>