drm/i915: check connector->encoder before using it.

DP MST will need connectors that aren't connected to specific
encoders, add some checks in advance to avoid oopses.

Reviewed-by: Todd Previte <tprevite@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --cc drivers/gpu/drm/i915/i915_debugfs.c
Simple merge

diff --cc drivers/gpu/drm/i915/i915_irq.c
Simple merge

diff --cc drivers/gpu/drm/i915/intel_display.c
Simple merge