drm/gma500: Stop using mode->private_flags

gma500 only uses mode->private_flags to convey the sdvo pixel
multiplier from the encoder .mode_fixup() hook to the encoder
.mode_set() hook. Those always seems get called as a pair so
let's just stuff the pixel multiplier into the encoder itself
as there are no state objects we could use in this non-atomic
driver.

Paves the way for nuking mode->private_flag.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
CC: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428171940.19552-15-ville.syrjala@linux.intel.com