drm/crc: Add support for polling on the data fd.

This will make it possible for userspace to know whether reading
will block, without blocking on the fd. This makes it possible to
drain all queued CRC's in blocking mode, without having to reopen
the fd.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180202142743.68527-1-maarten.lankhorst@linux.intel.com


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[mlankhorst: Use POLLIN|POLLRDNORM, based on Ville's suggestion]