drm/msm/dpu: dont_use IS_ERR_OR_NULL for encoder phys backends

drm/msm/dpu: dont_use IS_ERR_OR_NULL for encoder phys backends

The functions dpu_encoder_phys_foo_init() can not return NULL. Replace
corresponding IS_ERR_OR_NULL() checks with just IS_ERR().

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/485452/
Link: https://lore.kernel.org/r/20220507115942.1705872-1-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>