drm: Only take crtc lock in get_gamma ioctl

We don't call into drivers at all here, this is enough. Also, we can
reduce the critical section a bit to simplify the code.
crtc->gamma_size is set up once at driver load and then invariant, so
also doesn't need any protection.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403083304.9083-8-daniel.vetter@ffwll.ch