drm/msm/dpu: gracefully handle null fb commits for writeback

kms_writeback test cases also verify with a null fb for the
writeback connector job. In addition there are also other
commit paths which can result in kickoffs without a valid
framebuffer like while closing the fb which results in the
callback to drm_atomic_helper_dirtyfb() which internally
triggers a commit.

Add protection in the dpu driver to ensure that commits for
writeback encoders without a valid fb are gracefully skipped.

changes in v2:
	- rename dpu_encoder_has_valid_fb to dpu_encoder_is_valid_for_commit

changes in v3:
	- none

Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/483522/
Link: https://lore.kernel.org/r/1650984096-9964-17-git-send-email-quic_abhinavk@quicinc.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>