drm/panfrost: Handle IDVS_GROUP_SIZE feature

The IDVS group size feature was missing. It is used on some Bifrost and
Valhall GPUs, and is the last kernel-relevant Bifrost feature we're
missing.

This feature adds an extra IDVS group size field to the JM_CONFIG
register. In kbase, the value is configurable via the device tree; kbase
uses 0xF as a default if no value is specified. Until we find a device
demanding otherwise, let's always set the 0xF default on devices which
support this feature mimicking kbase's behaviour.

Tuning this register slightly improves performance of index-driven
vertex shading. On Mali-G52 (with Mesa), overall glmark2 score is
improved from 1026 to 1037. Geometry-heavy scenes like -bshading are
improved from 1068 to 1098.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220211145849.3148-1-alyssa.rosenzweig@collabora.com