drm/atmel-hlcdc: Use crtc->mode_valid() callback

Now that we have a callback to check if crtc supports a given mode
we can use it in atmel-hlcdc so that we restrict the number of probbed
modes to the ones we can actually display.

Also, remove the mode_fixup() callback as this is no longer needed
because mode_valid() will be called before.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: http://patchwork.freedesktop.org/patch/msgid/95fd6c06c58bd0b957e36a8d7068e6a74b581304.1495720737.git.joabreu@synopsys.com