intel/blorp: fix uninitialized variable warning

intel/blorp: fix uninitialized variable warning

Compiler doesn't pick up that level and start_layer will be defined,
so do as was done for num_layers in 4d8b476fa9a "intel/blorp: Fix
compiler warning about num_layers." and always set it.

Fixes warning

../../src/mesa/drivers/dri/i965/brw_blorp.c: In function ‘brw_blorp_clear_depth_stencil’:
../../src/mesa/drivers/dri/i965/brw_blorp.c:1439:4: warning: ‘start_layer’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    blorp_clear_depth_stencil(&batch, &depth_surf, &stencil_surf,
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                              level, start_layer, num_layers,
                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                              x0, y0, x1, y1,
                              ~~~~~~~~~~~~~~~
                              (mask & BUFFER_BIT_DEPTH), ctx->Depth.Clear,
                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                              stencil_mask, ctx->Stencil.Clear);
                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../src/mesa/drivers/dri/i965/brw_blorp.c:1439:4: warning: ‘level’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>