drm/panfrost: increase readl_relaxed_poll_timeout values

Amlogic SoC devices report the following errors frequently causing excessive
dmesg log spam and early log rotataion, although the errors appear to be
harmless as everything works fine:

[    7.202702] panfrost ffe40000.gpu: error powering up gpu L2
[    7.203760] panfrost ffe40000.gpu: error powering up gpu shader

ARM staff have advised increasing the timeout values to eliminate the errors
in most normal scenarios, and testing with several different G31/G52 devices
shows 20000 to be a reliable value.

Fixes: f3ba9122 ("drm/panfrost: Add initial panfrost driver")
Suggested-by: Steven Price <steven.price@arm.com>
Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201008141738.13560-1-christianshewitt@gmail.com