drm/vc4: Allow vblank_disable_immediate on non-fw-kms. (v2)

With instantaneous high precision vblank timestamping
that updates at leading edge of vblank, the emulated
"hw vblank counter" from vblank timestamping which
increments at leading edge of vblank, and reliable
page flip execution and completion at leading edge
of vblank, we should meet the requirements for fast
vblank irq disable/enable.

Testing against rpi-4.12-rc5 Linux kernel with timing
measurement equipment indicates this works fine,
so allow immediate vblank disable for power saving.

For debugging in case of unexpected trouble, booting
with kernel cmdline option drm.vblankoffdelay=0
would keep vblank irqs on to approximate old behavior.

v2: Respin onto drm-misc-next, per Eric's suggestion.
Drop !vc4->firmware_kms check, as the firmware_kms
implementation does not exist in upstream.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170622012811.2139-1-mario.kleiner.de@gmail.com