OMAPDSS: fix build when DEBUG_FS or DSS_DEBUG_SUPPORT disabled

If CONFIG_DEBUG_FS or CONFIG_OMAP2_DSS_DEBUG_SUPPORT is disabled, the
build fails:

drivers/video/omap2/dss/core.c:197:50: error: static declaration of
'dss_debugfs_create_file' follows non-static declaration
drivers/video/omap2/dss/dss.h:166:5: note: previous declaration of
'dss_debugfs_create_file' was here

This patch fixes the dummy dss_debugfs_create_file() so that the driver
builds.

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