drm/msm: split the main platform driver

Currently the msm platform driver is a multiplex handling several cases:
- headless GPU-only driver,
- MDP4 with flat device nodes,
- MDP5/DPU MDSS with all the nodes being children of MDSS node.

This results in not-so-perfect code, checking the hardware version
(MDP4/MDP5/DPU) in several places, checking for mdss even when it can
not exist, etc. Split the code into three handling subdrivers (mdp4,
mdss and headless msm).

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/482507/
Link: https://lore.kernel.org/r/20220419155346.1272627-4-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>