gpu: drm: i915: compress logic into one line

Simplify logic to avoid unnecessary variable declaration and assignment.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170515220028.GA15149@embeddedgus