slv flush callback is wrong

[AndreCostaaa]

Even when using DIRECT render mode with a buffer size equal to framebuffer for LVGL, the flush callback is still called multiple times.
The current flush callback will flush the display too many times causing a huge slowdown

so3/usr/lib/slv/slv_fb.c

Lines 135 to 140 in cc8fba7

	static void my_fb_cb(lv_display_t *disp, const lv_area_t *area, uint8_t *px_map)
	{
	slv_fb_priv_t *priv = (slv_fb_priv_t *)lv_display_get_user_data(disp);
	memcpy(priv->fbp, px_map, priv->fb_size);
	lv_display_flush_ready(disp);
	}