omapfb: lcd_ams_delta: add support for contrast control

The patch extends the Amstrad Delta LCD panel driver with optional support for
changing contrast using standard LCD class device API instead of setting it
silently to a default value at panel enable. It also allows for lowering power
consumption by turning off OMAP_PWL_CLK_ENABLE via lcd_ops.set_power callback.

Created and tested against linux-omap for-next,
commit 155a75d9725e66e5ec8a383822957dee52427057.

Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>