media: include/video/omapfb_dss.h: use IS_ENABLED()

Just checking for ifdefs cause build issues as reported by
kernel test:

config: openrisc-allmodconfig (attached as .config)
compiler: or1k-linux-gcc (GCC) 6.0.0 20160327 (experimental)

All errors (new ones prefixed by >>):

   drivers/video/fbdev/omap2/omapfb/omapfb-main.c: In function 'omapfb_init_connections':
>> drivers/video/fbdev/omap2/omapfb/omapfb-main.c:2396:8: error: implicit declaration of function 'omapdss_find_mgr_from_display' [-Werror=implicit-function-declaration]
     mgr = omapdss_find_mgr_from_display(def_dssdev);
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   drivers/video/fbdev/omap2/omapfb/omapfb-main.c:2396:6: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
     mgr = omapdss_find_mgr_from_display(def_dssdev);
         ^
   drivers/video/fbdev/omap2/omapfb/omapfb-main.c: In function 'omapfb_find_default_display':
>> drivers/video/fbdev/omap2/omapfb/omapfb-main.c:2430:13: error: implicit declaration of function 'omapdss_get_default_display_name' [-Werror=implicit-function-declaration]
     def_name = omapdss_get_default_display_name();
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   drivers/video/fbdev/omap2/omapfb/omapfb-main.c:2430:11: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
     def_name = omapdss_get_default_display_name();
              ^

So, use IS_ENABLED() instead.

Fixes: 771f7be8 ("media: omapfb: omapfb_dss.h: add stubs to build with COMPILE_TEST && DRM_OMAP")

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: tomi.valkeinen@ti.com
Cc: linux-omap@vger.kernel.org
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

include/video/omapfb_dss.h

@@ -778,7 +778,7 @@ struct omap_dss_driver {
 
 typedef void (*omap_dispc_isr_t) (void *arg, u32 mask);
 
-#ifdef CONFIG_FB_OMAP2
+#if IS_ENABLED(CONFIG_FB_OMAP2)
 
 enum omapdss_version omapdss_get_version(void);
 bool omapdss_is_initialized(void);