drm: simplify the locking in the GETCRTC ioctl

No need to grab both plane and crtc locks at the same time, we can do
them one after the other. If userspace races it'll get what it
deserves either way.

This removes another user of drm_modeset_lock_crtc. There's only one
left.

v2: Make sure all access to primary->state is properly protected
(Harry).

Cc: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170328070145.21520-1-daniel.vetter@ffwll.ch