drm/panfrost: Add a no execute flag for BO allocations

Executable buffers have an alignment restriction that they can't cross
16MB boundary as the GPU program counter is 24-bits. This restriction is
currently not handled and we just get lucky. As current userspace
assumes all BOs are executable, that has to remain the default. So add a
new PANFROST_BO_NOEXEC flag to allow userspace to indicate which BOs are
not executable.

There is also a restriction that executable buffers cannot start or end
on a 4GB boundary. This is mostly avoided as there is only 4GB of space
currently and the beginning is already blocked out for NULL ptr
detection. Add support to handle this restriction fully regardless of
the current constraints.

For existing userspace, all created BOs remain executable, but the GPU
VA alignment will be increased to the size of the BO. This shouldn't
matter as there is plenty of GPU VA space.

Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808222200.13176-6-robh@kernel.org