u_vbuf: fix potentially bogus assert

u_vbuf: fix potentially bogus assert

There are cases where we hit u_vbuf path due to alignment or pitch-
alignment restrictions, but for an output-format that u_vbuf does not
support translating (yet the driver does support natively).  In which
case we hit the memcpy() path and don't care that u_vbuf doesn't
understand it.

Fixes crash with debug build of mesa in:
dEQP-GLES3.functional.vertex_arrays.single_attribute.strides.fixed.user_ptr_stride17_components2_quads1

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95000
Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>