mesa: stop using _NEW_STENCIL with st/mesa, use DriverFlags.NewStencil instead

mesa: stop using _NEW_STENCIL with st/mesa, use DriverFlags.NewStencil instead

This bypasses _mesa_update_state_locked.

Before:
   DrawElements ( 1 VBOs, 4 UBOs,  8 Tex) w/ stencil enable change:    3.99 million
   DrawArrays   ( 1 VBOs, 4 UBOs,  8 Tex) w/ stencil enable change:    4.56 million

After:
   DrawElements ( 1 VBOs, 4 UBOs,  8 Tex) w/ stencil enable change:    4.93 million
   DrawArrays   ( 1 VBOs, 4 UBOs,  8 Tex) w/ stencil enable change:    5.84 million

It's quite a difference in the draw call rate when ctx->NewState stays
equal to 0 the whole time.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>