drm/i915: remove irrelevant DRM_UNLOCKED flag

DRM_UNLOCKED doesn't do anything for non-legacy drivers. Remove it.

Cc: intel-gfx@lists.freedesktop.org
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190522154702.16269-3-emil.l.velikov@gmail.com