media: mtk-vcodec: vdec: remove redundant 'pfb' assignment

A clang-analyzer warning was reported.

>> drivers/media/platform/mtk-vcodec/mtk_vcodec_dec_stateless.c:133:18:
   warning: Value stored to 'pfb' during its initialization is never
   read [clang-analyzer-deadcode.DeadStores]

            struct vdec_fb *pfb = &framebuf->frame_buffer;
                            ^~~   ~~~~~~~~~~~~~~~~~~~~~~~

Removes the redundant 'pfb' assignment to fix.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>