drm/i915/selftests: Fix MI_STORE_DWORD_IMM alignment

MI_STORE_DWORD_IMM wants to write into a dword-aligned (4B) address, we
mistakenly cleared bit2 and not bits 0 and 1.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190306082447.21563-1-chris@chris-wilson.co.uk