OMAP: DSS2: HDMI: make set_timing saner

Currently the set_timings code for hdmi is quite strange. The display is
disabled in hdmi_omap4_panel.c before setting timings, and enabled in
hdmi.c after setting the timings. Furthermore, the timings were not
permanent, and disabling and enabling the display would lose them.

This patch makes the set_timings handling a bit better.

Cc: Mythri P K <mythripk@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>