panfrost: Enable helper invocations when texturing

panfrost: Enable helper invocations when texturing

it turns out we have explicit control over helper invocations; if a
particular bit in the fragment shader descriptor is set, helper
invocations are launched; if it clear, they are not. Helper invocations
are required whenever computing derivatives, whether explicitly
(dFdx/dFdy) *or* implicitly (any texturing). Accordingly, we set this
bit when texturing to fix edge case behaviour (literally, haha).

Thank you to Jason Ekstrand and Ilia Mirkin for pointing out the
representative dEQP test failed along triangle edges and for suggesting
helper invocations / derivatives as a list of suspect pieces (which led
to discovering the helper invocations enable bit in the first place).

Ideally we would use the new NIR analysis pass for this, but that hasn't
landed quite yet.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>