modetest: add support for parsing big-endian formats

modetest: add support for parsing big-endian formats

When specifying a frame buffer format like "RG16_BE" (big-endian RG16),
modetest still uses the little-endian variant, as the format string is
truncated to four characters.

Fix this by increasing the format string size to 8 bytes (7 characters +
NUL terminator).

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

v4:
  - No changes,

v3:
  - Update for suffix change from "be" to "_BE", cfr. commit
    ffb9375a505700ad ("xf86drm: handle DRM_FORMAT_BIG_ENDIAN in
    drmGetFormatName()"),
  - Replace hardcoded numbers in code by sizeof(),

v2:
  - New.