drm/blend: fix typo in the comment

The documentation for drm_rotation_simplify() uses DRM_MODE_ROTATE_X,
while it's clear the comment should mention DRM_MODE_REFLECT_X
instead. The example passes all flags except the DRM_MODE_REFLECT_X as
supported and expects to eliminate this flag.

Fixes: c2c446ad ("drm: Add DRM_MODE_ROTATE_ and DRM_MODE_REFLECT_ to UAPI")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220316074648.7009-1-dmitry.baryshkov@linaro.org