st/mesa: fix a bug in and re-org setup_interleaved_attribs()

st/mesa: fix a bug in and re-org setup_interleaved_attribs()

We were mis-computing the size of the user-space vertex buffer in
some circumstances.  This led to a failed assertion at u_inlines.h:222
when using the VMware svga driver.

For example, if we had arrays such as:

array[0]: element_offset = 12, stride = 24
array[1]: element_offset = 0, stride = 24

We'd mistakenly compute 'bytes' to be 12 bytes too small.

I've reorganized the function too.  By time it's called, we know that
we've got interleaved arrays either all in one VBO or all in user memory
and the stride is equal for all arrays.

Move the code that lived inside the attr==0 test after the loop.

In the loop we compute the true vertex size.  That size factors into the
pipe->redefine_user_buffer() call later.  Using the vertex size instead
of array[0]'s element_offset fixes the failed assertion.

Reviewed-by: José Fonseca <jfonseca@vmware.com>