drm/mgag200: Introduce separate variable for PLL S parameter

drm/mgag200: Introduce separate variable for PLL S parameter

The S parameter is controls the loop filter bandwidth when programming
the PLL. It's currently stored as part of P (i.e., the clock divider.)

Add a separate variable for S prepares the PLL code for further
refactoring. The value of s is currently 0, so it has not yet an effect
on the programming.

v2:
	* add a note on the current value of s to commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714142240.21979-6-tzimmermann@suse.de