drm: arcpgu: Use crtc->mode_valid() callback

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

This is specially useful because arcpgu crtc is responsible to set
a clock value in the commit() stage but unfortunatelly this clock
does not support all the needed ranges.

Also, remove the atomic_check() callback as mode_valid() callback
will be called before.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Reviewed-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Carlos Palminha <palminha@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@linux.ie>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Changes v4->v5:
	- Change commit message to "arcpgu" (Alexey)
Changes v3->v4:
	- Do not use aux function (Laurent)
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/c3bcd69016c77f68a03ff3cb6b22ca6f90e930b0.1495720737.git.joabreu@synopsys.com