anv/image: Set different usage flags for shadow surfaces

anv/image: Set different usage flags for shadow surfaces

For the block BLOCK_TEXEL_VIEW_COMPATIBLE case, this didn't matter
because the flags were already more-or-less what we wanted.  However,
for gen7 stencil shadow images, it still had ISL_SURF_USAGE_STENCIL_BIT
so we were getting W-tiled which isn't what we want for the shadow.  By
passing just ISL_SURF_USAGE_TEXTURE_BIT (and CUBE if we care), we now
get something that's actually texturable.

Fixes: f3ea0cf828 "anv: Add stencil texturing support for gen7"