drm/fsl-dcu: use flat regmap cache

Using flat regmap cache instead of RB-tree to avoid the following
lockdep warning on driver load:
WARNING: CPU: 0 PID: 1 at kernel/locking/lockdep.c:2755 lockdep_trace_alloc+0x15c/0x160()
DEBUG_LOCKS_WARN_ON(irqs_disabled_flags(flags))

The RB-tree regmap cache needs to allocate new space on first
writes. However, allocations in an atomic context (e.g. when a
spinlock is held) are not allowed. The function regmap_write
calls map->lock, which acquires a spinlock in the fast_io case.
Since the FSL DCU driver uses MMIO, the regmap bus of type
regmap_mmio is being used which has fast_io set to true.

Use flat regmap cache and specify max register to be large
enouth to cover all registers available in LS1021a and Vybrids
register space.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Cc: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org