drm/msm/dpu: drop wb2_formats_rgb

After enabling YUV support for writeback on a variety of DPU hardware,
the wb2_formats_rgb is now unused. Drop it following the report of LKP.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202503071857.oZbQsPaE-lkp@intel.com/
Reviewed-by: Rob Clark <robdclark@gmail.com> # on IRC
Patchwork: https://patchwork.freedesktop.org/patch/641848/
Link: https://lore.kernel.org/r/20250308-dpu-drop-wb2-rgb-v1-1-f5503fcd1bc2@linaro.org


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