drm/i915/dsb: Avoid corrupting the first register write

i915_gem_object_create_internal() does not hand out zeroed
memory. Thus we may confuse whatever stale garbage is in
there as a previous register write and mistakenly handle the
first actual register write as an indexed write. This can
end up corrupting the instruction sufficiently well to lose
the entire register write.

Make sure we've actually emitted a previous instruction before
attemting indexed register write merging.

Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Animesh Manna <[email protected]>

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