radeonsi: Do colorformat endian swap for PIPE_USAGE_STAGING

radeonsi: Do colorformat endian swap for PIPE_USAGE_STAGING

There is an old if statement (dated to 2011) that prevented doing
endian swap for colorformat, in case the buffer is marked as
PIPE_USAGE_STAGING.

This is now wrong because st_ReadPixels() reads into a destination
texture that is marked with PIPE_USAGE_STAGING. Therefore, even if
the texture is rendered correctly to the monitor, when reading it
back we get unswapped/wrong values.

This patch makes the check_rgba() function in gl-1.0-readpixsanity
piglit test pass in big-endian.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>