drm/i915: add module param "enable_dp_mst"

Adds an (unsafe; auto-kernel-tainting) boolean module parameter to the i915
drm driver: "enable_dp_mst", which is enabled by default.  Disabling the
parameter forces newly connected DisplayPort sinks to report as not
supporting multi-stream transport (MST), thus "forcing" the use of
single-stream transport (SST).

v2: rename parameter to conform to style
v3: add signoff

Signed-off-by: Nathan Schulte <nmschulte@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1458054845-5837-1-git-send-email-nmschulte@gmail.com