drm/blend: Fix comment typ-o

Fix DRM_REFELCT_Y -> DRM_REFLECT_Y.

Signed-off-by: Robert Foss <robert.foss@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170519205017.23307-1-robert.foss@collabora.com