drm/armada: fix overlay when partially off-screen

Fix the start address calculation when overlay is partially off screen.
fb->bits_per_pixel is not set for YUV formats, and is always zero, which
led to the first component always starting at zero.

Use drm_format_plane_cpp() instead.

This also revealed a problem in that YUYV formats toggle the U/V data
for odd pixel start address offsets.  We try to rectify that by
toggling the U/V swap, which for the most part works, but seemingly
introduces a flicker for one scan frame of swapped U/V.

However, these changes result in an overall improvement.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>