gma500: Discard modes that don't fit in stolen memory

[This fixes a crash on boot if the system is plugged into an HDTV so it's
 probably appropriate to push even though it didn't make the window. We could
 be cleverer about this but the simple version seems to be the safe one]

From: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>

At the moment we cannot allocate more than stolen memory size for framebuffers.
To get around that issues we discard modes that doesn't fit. This is a temporary
solution until we can freely allocate framebuffer memory.

[Currently the framebuffer needs to be linear in kernel space due to limits
 in the kernel fb layer - AC]

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>