i915: Use Stencil.Enabled instead of Stencil._Enabled in DrawBuffers.

The _Enabled field isn't updated at the point that DrawBuffers is called,
and the Driver.Enable() function does the testing for stencil buffer
presence anyway.

bug #21608 for Radeon
(cherry picked from commit 4c6f82989983eecc0b3b724716cb3bcb675664c5)
1 file changed