drm/vc4: Remove broken attempt at GPU reset using genpd.

I've tested and confirmed that it doesn't actually work.  We'll need
to sort out how to do this properly later, but for now just remove it
since it also caused build breakage due to using CONFIG_PM_SLEEP
functions without our Kconfig depending on PM_SLEEP.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>