compositor-drm: Fix refresh rate selection in drm_output_switch_mode

compositor-drm: Fix refresh rate selection in drm_output_switch_mode

The matching logic in choose_mode() compared refresh rate
of a drm_mode candidate mode expressed in Hz against the
requested refresh rate of the target weston_mode expressed
in milliHz, so the match always failed and the logic always
ended up the mode with the highest refresh rate for a given
resolution, instead of the one matching the requested rate.

Match proper fields to fix this.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>