drm: Validate requested virtual size against allocated fb size

drm: Validate requested virtual size against allocated fb size

commit 62fb376e214d3c1bfdf6fbb77dac162f6da04d7e upstream.

mplayer -vo fbdev tries to create a screen that is twice as tall as the
allocated framebuffer for "doublebuffering". By default, and all in-tree
users, only sufficient memory is allocated and mapped to satisfy the
smallest framebuffer and the virtual size is no larger than the actual.
For these users, we should therefore reject any userspace request to
create a screen that requires a buffer larger than the framebuffer
originally allocated.

References: https://bugs.freedesktop.org/show_bug.cgi?id=38138
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>