drm/vc4: Let gpiolib know that we're OK with sleeping for HPD.

Fixes an error thrown every few seconds when we poll HPD when it's on
a I2C to GPIO expander.

Signed-off-by: Eric Anholt <eric@anholt.net>
Tested-by: Daniel Stone <daniels@collabora.com>
(cherry picked from commit 0e60eab5)