drm: atmel-hlcdc: use appropriate enabled flag in suspend/resume

Unfortunately we used the enabled flag in struct drm_crtc instead of the
enabled flag in struct atmel_hlcdc_crtc. This obviously leads to
discrepancies on crtc enable state.

This patch fixes the issue by using the struct atmel_hlcdc_crtc enabled
flag in PM support.

Signed-off-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>