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 <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Luben Tuikov <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/atombios_encoders.c

···
 	struct backlight_properties props;
 	struct amdgpu_backlight_privdata *pdata;
 	struct amdgpu_encoder_atom_dig *dig;
-	u8 backlight_level;
 	char bl_name[16];
 
 	/* Mac laptops with multiple GPUs use the gmux driver for backlight
···
 	}
 
 	pdata->encoder = amdgpu_encoder;
-
-	backlight_level = amdgpu_atombios_encoder_get_backlight_level_from_reg(adev);
 
 	dig = amdgpu_encoder->enc_priv;
 	dig->bl_dev = bd;