OMAPDSS: Fix compile for dss_debugfs_create_file

Otherwise we get:

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

Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>