drm/vmwgfx: Limit max desktop dimensions to 8Kx8K

This was originally chosen to be an arbitrarily large number.  However,
some user mode may actually try to set a 16Kx16K mode and run into other
issues.

Since 8Kx8K is the current texture limit for Mesa LLVM driver, we will
just use this limit for now.

Cc: <stable@vger.kernel.org> # 4.12.x
Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>