splash: display splash in DM_VIDEO configurations

Currently for CONFIG_DM_VIDEO=y setting splashimage env variable doesn't
have any effect. Introduce a common function for both dm-video/lcd stacks,
that checks env("splashimage") and invokes bmp_display() accordingly.
For additional details please check discussion [1].

[1] https://lists.denx.de/pipermail/u-boot/2019-May/371002.html

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
diff --git a/include/splash.h b/include/splash.h
index 228aff4..7fd2de8 100644
--- a/include/splash.h
+++ b/include/splash.h
@@ -66,10 +66,10 @@
 static inline void splash_get_pos(int *x, int *y) { }
 #endif
 
-#if defined(CONFIG_SPLASH_SCREEN) && defined(CONFIG_LCD)
-int lcd_splash(ulong addr);
+#if defined(CONFIG_SPLASH_SCREEN) && defined(CONFIG_CMD_BMP)
+int splash_display(void);
 #else
-static inline int lcd_splash(ulong addr)
+static inline int splash_display(void)
 {
 	return -ENOSYS;
 }