OMAPDSS: Improve regulator names for DT

The regulator names used for DSS components are somewhat ugly for DT
use. As we're just adding DT support, it's simple to change the
regulator names.

This patch makes the DSS driver get the regulators with somewhat cleaner
names when bootin with DT. For example, this allows us to define HDMI's
VDDA regulator in the DT data as:

vdda-supply = <...>;

instead of

vdda_hdmi_dac-supply = <...>;

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit@ti.com>