drm: Export the command-line mode parser

In the absence of configuration data for providing the fixed mode for
a panel, I would like to be able to pass such modes along a separate
module paramenter. To do so, I then need to parse a modeline from a
string, which drm is already capable of. Export that capability to the
drivers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>