media: xilinx: simplify get fourcc logic

Right now, there are two calls for xvip_get_format_by_fourcc().
If the first one fails, it is called again in order to pick
the first available format: V4L2_PIX_FMT_YUYV.

This ends by producing a smatch warnings:
	drivers/media/platform/xilinx/xilinx-dma.c:555 __xvip_dma_try_format() error: 'info' dereferencing possible ERR_PTR()
	drivers/media/platform/xilinx/xilinx-dma.c: drivers/media/platform/xilinx/xilinx-dma.c:664 xvip_dma_init() error: 'dma->fmtinfo' dereferencing possible ERR_PTR()

as it is hard for an static analyzer to ensure that calling
xvip_get_format_by_fourcc(XVIP_DMA_DEF_FORMAT) won't return an
error.

So, better to optimize the logic, ensuring that the function
will never return an error.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>