drm/radeon/dpm: fix display_gap programming on rv7xx

Check the driver state rather than the register as the
crtc registers may not be enabled yet.

Should fix:
https://bugzilla.kernel.org/show_bug.cgi?id=60510
https://bugs.freedesktop.org/show_bug.cgi?id=66651



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>