drm: adv7511: Fix low refresh rate register for ADV7533/5

For ADV7533 and ADV7535 low refresh rate is selected using
bits [3:2] of 0x4a main register.
So depending on ADV model write 0xfb or 0x4a register.

Fixes: 2437e7cd ("drm/bridge: adv7533: Initial support for ADV7533")
Reviewed-by: Robert Foss <rfoss@kernel.org>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Bogdan Togorean <bogdan.togorean@analog.com>
Signed-off-by: Alexandru Ardelean <alex@shruggie.ro>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230719060143.63649-1-alex@shruggie.ro