viafb: rename display_timing to via_display_timing

The struct display_timing is specific to the via subsystem. The naming leads to
collisions with the new struct display_timing, which is supposed to be a shared
struct between different subsystems.
To clean this up, prepend the existing struct with the subsystem it is specific
to.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>