drm/i915/cmdparser: Check for SKIP descriptors first

If the command descriptor says to skip it, ignore checking for anyother
other conflict.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-37-chris@chris-wilson.co.uk