svga: reset vertex buffer offset in svga_release_user_upl_buffers()

svga: reset vertex buffer offset in svga_release_user_upl_buffers()

This function releases the buffer that contains user-space vertex data.
The buffer_offset field points into that buffer.  So reset the
buffer_offset to zero when we release the buffer so that subsequent
draws don't inadvertantly get a bad offset.

Fixes error messages / failed assertions (in the draw module's bounds/size
checking code) when running piglit's polygon-mode test.

NOTE: This is a candidate for the 8.0 branch.

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