pinctrl: aspeed-g5: Delay acquisition of regmaps

While sorting out some devicetree issues I found that the pinctrl driver
was failing to acquire its GFX regmap even though the phandle was
present in the devicetree:

    [    0.124190] aspeed-g5-pinctrl 1e6e2000.syscon:pinctrl: No GFX phandle found, some mux configurations may fail

Without access to the GFX regmap we fail to configure the mux for the
VPO function:

    [    1.548866] pinctrl core: add 1 pinctrl maps
    [    1.549826] aspeed-g5-pinctrl 1e6e2000.syscon:pinctrl: found group selector 164 for VPO
    [    1.550638] aspeed-g5-pinctrl 1e6e2000.syscon:pinctrl: request pin 144 (V20) for 1e6e6000.display
    [    1.551346] aspeed-g5-pinctrl 1e6e2000.syscon:pinctrl: request pin 145 (U19) for 1e6e6000.display
    ...
    [    1.562057] aspeed-g5-pinctrl 1e6e2000.syscon:pinctrl: request pin 218 (T22) for 1e6e6000.display
    [    1.562541] aspeed-g5-pinctrl 1e6e2000.syscon:pinctrl: request pin 219 (R20) for 1e6e6000.display
    [    1.563113] Muxing pin 144 for VPO
    [    1.563456] Want SCU8C[0x00000001]=0x1, got 0x0 from 0x00000000
    [    1.564624] aspeed_gfx 1e6e6000.display: Error applying setting, reverse things back

This turned out to be a simple problem of timing: The ASPEED pinctrl
driver is probed during arch_initcall(), while GFX is processed much
later. As such the GFX syscon is not yet registered during the pinctrl
probe() and we get an -EPROBE_DEFER when we try to look it up, however
we must not defer probing the pinctrl driver for the inability to mux
some GFX-related functions.

Switch to lazily grabbing the regmaps when they're first required by the
mux configuration. This generates a bit of noise in the patch as we have
to drop the `const` qualifier on arguments for several function
prototypes, but has the benefit of working.

I've smoke tested this for the ast2500-evb under qemu with a dummy
graphics device. We now succeed in our attempts to configure the SoC's
VPO pinmux function.

Fixes: 7d29ed88

 ("pinctrl: aspeed: Read and write bits in LPC and GFX controllers")
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20190724080155.12209-1-andrew@aj.id.au
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>