drm/amd/amdgpu/atombios_encoders: Remove set but unused variable 'backlight_level'

Also removing the call to amdgpu_atombios_encoder_get_backlight_level_from_reg()
since, according to Alex Deucher, "We call it again below indirectly".

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

 drivers/gpu/drm/amd/amdgpu/atombios_encoders.c: In function ‘amdgpu_atombios_encoder_init_backlight’:
 drivers/gpu/drm/amd/amdgpu/atombios_encoders.c:174:5: warning: variable ‘backlight_level’ set but not used [-Wunused-but-set-variable]

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: Luben Tuikov <luben.tuikov@amd.com>
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>