i965/miptree: Init r8stencil_needs_update to false

i965/miptree: Init r8stencil_needs_update to false

The current behavior masked two bugs where the flag was not set to true
after modifying the stencil texture. One case was a regression
introduced with commit bdbb527a65fc729e7a9319ae67de60d03d06c3fd and
another was a bug in the depthstencil mapping code. These have since
been fixed.

To prevent such bugs from being masked in the future, initialize
r8stencil_needs_update to false.

v2: Keep the delayed allocation.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>