iris: Fix fast-clearing of depth via glClearTex(Sub)Image
If we clear depth only texture via glClearTex(Sub)Image
it may cause:
../src/intel/blorp/blorp_genX_exec.h:1554: blorp_emit_surface_states: Assertion `params->depth.enabled || params->stencil.enabled' failed.
due to clear_depth_stencil calling blorp_clear_depth_stencil when depth is already fast-cleared and there is no stencil.
Fixes piglit test: arb_clear_texture-depth
piglit!334 (merged)
Fixes: 51638cf1