drm/msm/dpu: check for valid hw_pp in dpu_encoder_helper_phys_cleanup

In the Linux kernel, the following vulnerability has been resolved:

drm/msm/dpu: check for valid hw_pp in dpu_encoder_helper_phys_cleanup

The commit 8b45a26f2ba9 ("drm/msm/dpu: reserve cdm blocks for writeback

in case of YUV output") introduced a smatch warning about another

conditional block in dpu_encoder_helper_phys_cleanup() which had assumed

hw_pp will always be valid which may not necessarily be true.

Lets fix the other conditional block by making sure hw_pp is valid

before dereferencing it.

Patchwork: https://patchwork.freedesktop.org/patch/574878/