OMAPDSS: HDMI: improve Makefile

We'll soon add support for OMAP5 HDMI, which uses some of the same files
as OMAP4 HDMI does.

This patch adds a new config entry "OMAP2_DSS_HDMI_COMMON", which both
OMAP4 and OMAP5 HDMI config entries can select. OMAP2_DSS_HDMI_COMMON
will cause the common HDMI files to be compiled.

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