iris: clear EXEC_OBJECT_WRITE flag in mask if bo not set writable

This fixes dEQP-EGL.functional.sharing.gles2.multithread.* tests that are hitting: "iris: Failed to submit batchbuffer: Invalid argument" error.

Closes: #5537 (closed) Fixes: e4c3d3ef ("iris: Defer construction of the validation (exec_object2) list") Signed-off-by: Tapani Pälli tapani.palli@intel.com