panfrost: Bump ESSL_FEATURE_LEVEL on Valhall

This advertises ARB_gpu_shader5 on Valhall, which should be working now. On the
GLES3.1 side, this notably adds support for sample variables and dynamic offsets
for texture gathers, both of which should now be working.

No shader-db changes.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!17101>
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