drm/fb: Improve drm_framebuffer.offsets documentation

Fix various spelling and grammar mistakes in the kerneldoc comments
documenting the offsets member in the drm_framebuffer structure:
  - s/laytou/layout/,
  - Add missing "is",
  - s/it/its/.

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/33fda13b500b39645e7363806c6e458e915b581e.1657113304.git.geert@linux-m68k.org