r600g: build fetch shader from vertex elements

r600g: build fetch shader from vertex elements

Vertex elements change are less frequent than draw call, those to
avoid rebuilding fetch shader to often build the fetch shader along
vertex elements. This also allow to move vertex buffer setup out
of draw path and make update to it less frequent.

Shader update can still be improved to only update SPI regs (based
on some rasterizer state like flat shading or point sprite ...).

Signed-off-by: Jerome Glisse <jglisse@redhat.com>