omapdrm: hdmi4: make low-level functions available

Three low-level functions in hdmi4.c and hdmi4_core.c are
made available for use by the OMAP4 CEC support.

Renamed the prefix to hdmi4 since these are OMAP4 specific.

These function deal with the HDMI core and are needed to
power it up for use with CEC, even when the HPD is low.

Background: even if the HPD is low it should still be possible
to use CEC. Some displays will set the HPD low when they go into standby or
when they switch to another input, but CEC is still available and able
to wake up/change input for such a display.

This is explicitly allowed by the CEC standard.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>