drm/panfrost: Don't set L2_MMU_CONFIG quirks

L2_MMU_CONFIG is an implementation-defined register. Different Mali GPUs
define slightly different MAX_READS and MAX_WRITES fields, which
throttle outstanding reads and writes when set to non-zero values. When
left as zero, reads and writes are not throttled.

Both kbase and panfrost always zero these registers. Per discussion with
Steven Price, there are two reasons these quirks may be used:

1. Simulating slower memory subsystems. This use case is only of
   interest to system-on-chip designers; it is not relevant to mainline.

2. Working around broken memory subsystems. Hopefully we never see this
   case in mainline. If we do, we'll need to set this register based on
   an SoC-compatible, rather than generally matching on the GPU model.

To the best of our knowledge, these fields are zero at reset, so the
write is not necessary. Let's remove the write to aid porting to new
Mali GPUs, which have different layouts for the L2_MMU_CONFIG register.

Suggested-by: Steven Price <steven.price@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220525145754.25866-8-alyssa.rosenzweig@collabora.com

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