drm: pl111: Simplify vexpress init

The init VExpress variants currently instantiates a 'muxfpga' driver for
the sole purpose of getting a regmap for it. There's no reason to
instantiate a driver and doing so just complicates things. The muxfpga
driver also isn't unregistered properly on module unload. Let's
just simplify all this this by just calling
devm_regmap_init_vexpress_config() directly.

Cc: Eric Anholt <eric@anholt.net>
Cc: dri-devel@lists.freedesktop.org
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200409013947.12667-3-robh@kernel.org