ARM: OMAP2+: Include soc.h to drm.c to fix compiling

I needed this when compiling for pandaboard.

The reason this is needed is that we are now using
omap_initcalls to keep them multiplatform safe.

Signed-off-by: André Hentschel <nerv@dawncrow.de>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Olof Johansson <olof@lixom.net>