drm/msm/dpu: inline dpu_encoder_get_wb()

drm/msm/dpu: inline dpu_encoder_get_wb()

The function dpu_encoder_get_wb() returns controller_id if the
corresponding WB is present in the catalog. We can inline this function
and rely on dpu_rm_get_wb() returning NULL for indices for which the
WB is not present on the device.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Tested-by: Abhinav Kumar <quic_abhinavk@quicinc.com> # sc7280
Patchwork: https://patchwork.freedesktop.org/patch/540634/
Link: https://lore.kernel.org/r/20230601172236.564445-5-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>