intel/blorp: initialize BLEND_STATE using braced initializer list

intel/blorp: initialize BLEND_STATE using braced initializer list

Coverity complains that memset has no effect, because of size 0.
Size of BLEND_STATE struct is 0 on gfx [6, 7.5], so memset has
nothing to do there. This is of course harmless, but we can make
code simpler by replacing memset with an empty initializer list
and at the same time avoid a warning from Coverity.

CID: 1486015

Signed-off-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11574>