OMAPDSS: HDMI: split power_on/off to two parts

There's currently just one power-on function for HDMI, which enables the
IP and the video output. When reading EDID or detecting if a monitor is
connected, we don't need the video output.

Enabling the video output for these operations is not a big problem in
itself, but the quick enable/disable cycles caused by the operations
seem to cause sync lost errors from time to time. Also, this makes it
possible to read the EDID before the full video path has been set up.

This patch splits the hdmi_power_on into two parts, hdmi_power_on_core
and hdmi_power_on_full. The "full" version does what hdmi_power_on does
currently, and hdmi_power_on_core only enables the core IP. Similar
changes are made for power_off.

Note that these don't allow the HDMI IP to be first enabled, and later
enable the video output, but the HDMI IP will first need to be powered
off before calling the full version. So this is rather limited
implementation, but it fills the needs for reading EDID.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Ricardo Neri <ricardo.neri@ti.com>