[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/sdmmc.c

@@ -174,8 +174,7 @@ static int _sdmmc_storage_readwrite(sdmmc_storage_t *storage, u32 sector, u32 nu
 			else
 				retries--;
 
-			sleep(500000);
-
+			sleep(100000);
 		} while (retries);
 		return 0;