[DP/GFX/SDMMC] Bugfixes

- Sanitize framebuffer. Original idea from @StevenMattera
- Change clear framebuffer to memset (for grey colors), for performance (we don't use alpha blending, so it does not matter to set it to 0xFF).
- Enable screen backlight after gfx init
- Reduce wait time for emmc/sd read/write retries to 100ms.

ipl/di.h

@@ -341,6 +341,9 @@ void display_end();
 /*! Show one single color on the display. */
 void display_color_screen(u32 color);
 
+/*! Switches screen backlight ON/OFF. */
+void display_backlight(u8 enable);
+
 /*! Init display in full 1280x720 resolution (32bpp, line stride 768, framebuffer size = 1280*768*4 bytes). */
 u32 *display_init_framebuffer();