drm/radeon/radeon_encoders: Move 'radeon_atom_backlight_init's prototype to shared header

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

 drivers/gpu/drm/radeon/atombios_encoders.c:186:6: warning: no previous prototype for ‘radeon_atom_backlight_init’ [-Wmissing-prototypes]
 186 | void radeon_atom_backlight_init(struct radeon_encoder *radeon_encoder,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~

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>