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 <61b5a29db2c0650f7c8510f93eece059c1b09320@vmware.com>

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