[media] mtk-vcodec: fix build warnings without DEBUG

After removing DEBUG from mtk_vcodec_util.h, some build warnings are
generated as the following:
.../drivers/media/platform/mtk-vcodec/vdec_vpu_if.c: In function 'vcodec_vpu_send_msg':
.../drivers/media/platform/mtk-vcodec/vdec_vpu_if.c:73:11: warning: unused variable 'msg_id' [-Wunused-variable]
  uint32_t msg_id = *(uint32_t *)msg;
           ^
.../drivers/media/platform/mtk-vcodec/mtk_vcodec_dec.c: In function 'vb2ops_vdec_buf_queue':
.../drivers/media/platform/mtk-vcodec/mtk_vcodec_dec.c:1129:7: warning: unused variable 'log_level' [-Wunused-variable]
   int log_level = ret ? 0 : 1;
       ^
.../drivers/media/platform/mtk-vcodec/venc_vpu_if.c: In function 'vpu_enc_send_msg':
.../drivers/media/platform/mtk-vcodec/venc_vpu_if.c:82:12: warning: unused variable 'msg_id' [-Wunused-variable]
   uint32_t msg_id = *(uint32_t *)msg;
            ^

It is because mtk_vcodec_debug() and mtk_vcodec_err() are defined as empty
macros. Without DEBUG definition, the variable for debugging is not used
anymore. Fixing build warnings by moving the assignment of the
variable to the argument of mtk_vcodec_debug() and mtk_vcodec_err().
Within the patch, build pass with/without DEBUG definition, and functions
still work fine.

Signed-off-by: Minghsiu Tsai <minghsiu.tsai@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>