drm: Fix typo in encoder docs

Corrected typo in bridge and encoder comparison. Also, added a one-line
encoder description from the previous documentation.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Archit Taneja <architt@codeaurora.org>

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1474324848-6446-1-git-send-email-dhinakaran.pandiyan@intel.com