drm/msm: remove extra indirection for msm_mdss

Since now there is just one mdss subdriver, drop all the indirection,
make msm_mdss struct completely opaque (and defined inside msm_mdss.c)
and call mdss functions directly.

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/482505/
Link: https://lore.kernel.org/r/20220419155346.1272627-3-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>