util: fix 16 bpp patterns on big-endian

util: fix 16 bpp patterns on big-endian

DRM formats are defined to be little-endian, unless the
DRM_FORMAT_BIG_ENDIAN flag is set.  Hence writes of multi-byte pixel
values need to take endianness into account.

Introduce a swap16() helper to byteswap 16-bit values, and a
cpu_to_le16() helper to convert 16-bit values from CPU-endian to
little-endian, and use the latter in the various pattern fill functions
for 16-bit formats.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
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v5:
  - Add Reviewed-by,

v4:
  - No changes,

v3:
  - Increase indentation after definition of cpu_to_le16(),

v2:
  - New.