drm/radeon/radeon_atombios: Move 'radeon_add_atom_encoder()'s prototype to shared header

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

 drivers/gpu/drm/radeon/atombios_encoders.c:2721:1: warning: no previous prototype for ‘radeon_add_atom_encoder’ [-Wmissing-prototypes]
 2721 | radeon_add_atom_encoder(struct drm_device *dev,
 | ^~~~~~~~~~~~~~~~~~~~~~~

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: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>