drm/vmwgfx: Refactor vmw_gb_surface_define_ioctl()

Refactored vmw_gb_surface_define_ioctl() and made the surface
definition part a separate function.  This way other parts of vmwgfx
can use it to allocate kernel-visible GB surfaces.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>