OMAP: DSS2: Delay regulator_get() calls

DSS submodules DPI/SDI/DSI/VENC require a regulator to function.
However, if the board doesn't use, say, SDI, the board shouldn't need to
configure vdds_sdi regulator required by the SDI module.

Currently the regulators are acquired when the DSS driver is loaded.
This means that if the kernel is configured with SDI, vdds_sdi regulator
is needed for all boards.

This patch changes the DSS driver to acquire the regulators only when a
display of particular type is initialized. For example, vdds_sdi is
acquired when sdi_init_display() is called.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>