[media] exynos-gsc: fix supported RGB pixel format

The driver exposes 32-bit A/XRGB 8-8-8-8 as supported format but testing
shows that using this format produces frames with wrong colors. The test
was done with the following GStreamer pipeline:

$ gst-launch-1.0 videotestsrc num-buffers=20 ! video/x-raw,format=UYVY \
! v4l2video3convert ! video/x-raw,format=xRGB ! videoconvert ! kmssink

The manual seems to state that the Pixel Format are in Little Endianness
so instead use the 32-bit BGRA/X 8-8-8-8 pixel format. This format works
correctly when using the following pipeline:

$ gst-launch-1.0 videotestsrc num-buffers=20 ! video/x-raw,format=UYVY \
! v4l2video3convert ! video/x-raw,format=BGRx ! kmssink

This change is similar to commit 7f2816e5 ("[media] s5p-fimc: Changed
RGB32 to BGR32") that fixed the same issue on a different Samsung driver.

Suggested-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>