drm: Clarify DRM_MODE_REFLECT_X/Y documentation

DRM_MODE_REFLECT_X and DRM_MODE_REFLECT_Y meaning seems a bit unclear
to me, so try to clarify that with a bit of ascii graphics.

Changes since v1:
  - Move the ascii graphics in the kerneldoc where all plane
    properties are already documented and make sure it's properly
    rendered, suggestested by Daniel Vetter.

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180910172946.18539-1-alexandru-cosmin.gheorghe@arm.com