drm/mode: Improve drm_mode_fb_cmd2 documentation

Fix various grammar mistakes in the kerneldoc comments documenting the
drm_mode_fb_cmd2 structure:
  - s/is/are/,
  - s/8 bit/8-bit/.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/536de72eab09242e1faf22fa58d91c9005d6ea51.1657113597.git.geert@linux-m68k.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220608020614.4098292-1-bjorn.andersson@linaro.org