drm/msm: move MDSS registers to separate header file

In preparation of adding more registers, move MDSS-related headers to
the separate top-level file.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/615310/
Link: https://lore.kernel.org/r/20240921-msm-mdss-ubwc-v1-1-411dcf309d05@linaro.org