nv30: reset the stencil mask when fast-clearing
nv30: reset the stencil mask when fast-clearing

Apparently the stencil mask applies to clears on nv30/nv40. Reset it to
0xff before doing a stencil clear. This fixes gl-1.0-readpixsanity and
a number of other piglit tests.

Signed-off-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler