svga: reduce unmapping/remapping of the default constant buffer

Previously, every time we put shader constants into the default constant
buffer we called u_upload_alloc(), which mapped the buffer, and
u_upload_unmap().  We had to unmap the buffer before calling
svga_buffer_handle() to get the winsys handle for the buffer.  But we
really only need to do that the first time we reference the const buffer.
Now we try to keep the upload manager's buffer mapped until we fill it or
flush the command buffer.

v2: add additional comment on the buffer unmapping code in
svga_context_flush(), per Charmaine.

Reviewed-by: Charmaine Lee <[email protected]>

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