drm/arm: malidp: Use crtc->mode_valid() callback

Now that we have a callback to check if crtc supports a given mode
we can use it in malidp 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>
Cc: Carlos Palminha <palminha@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@linux.ie>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Archit Taneja <architt@codeaurora.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Liviu Dudau <liviu@dudau.co.uk>