drm/gma500: Remove unused mrst_clock_funcs

drm/gma500: Remove unused mrst_clock_funcs

The mrst_clock_funcs const was added in 2013 by
commit ac6113ebb70d ("drm/gma500/mrst: Add SDVO clock calculation")
and commented as 'Not used yet'.

It's not been used since, so remove it.
The helper functions it points to are still used elsewhere.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306155155.212599-1-linux@treblig.org