drm/mgag200: Select clock in PLL update functions

Put the clock-selection code into each of the PLL-update functions to
make them select the correct pixel clock. Instead of copying the code,
introduce a new helper WREG_MISC_MASKED, which does masked writes into
<MISC>. Use it from each individual PLL update function.

The pixel clock for video output was not actually set before programming
the clock's values. It worked because the device had the correct clock
pre-set.

v2:
	* don't duplicate <MISC> update code (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: db05f8d3 ("drm/mgag200: Split MISC register update into PLL selection, SYNC and I/O")
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v5.9+
Link: https://patchwork.freedesktop.org/patch/msgid/20210714142240.21979-2-tzimmermann@suse.de