[media] mtk-mdp: fix compilation warnings if !DEBUG

The mtk_mdp_dbg() is empty if !DEBUG. This causes the following
warnings:

	drivers/media/platform/mtk-mdp/mtk_mdp_m2m.c: In function ‘mtk_mdp_try_fmt_mplane’:
	drivers/media/platform/mtk-mdp/mtk_mdp_m2m.c:231:52: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]
	        org_w, org_h, pix_mp->width, pix_mp->height);
                                                    ^
	drivers/media/platform/mtk-mdp/mtk_mdp_m2m.c: In function ‘mtk_mdp_m2m_start_streaming’:
	drivers/media/platform/mtk-mdp/mtk_mdp_m2m.c:414:21: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]
	        ctx->id, ret);
	                     ^

With could actually make the code to do something wrong. So,
add an empty block to make it be parsed ok.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>