drm/dp: Read the tx msg state once after checking for an event

Both as an exercise to document that we are reading the state outside of
the appropriate mutex and to ensure that we only read the value once
before the multiple comparisons, use READ_ONCE.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170513105201.17658-1-chris@chris-wilson.co.uk