drm/msm/dpu: call dpu_rm_get_intf() from dpu_encoder_get_intf()

drm/msm/dpu: call dpu_rm_get_intf() from dpu_encoder_get_intf()

There is little sense to get intf index just to call dpu_rm_get_intf()
on it. Move dpu_rm_get_intf() call to dpu_encoder_get_intf() function.

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/540632/
Link: https://lore.kernel.org/r/20230601172236.564445-6-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>