drm/v3d: Make sure the GPU is on when measuring clocks.

You'll get garbage measurements if the registers always read back
0xdeadbeef

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190220233658.986-3-eric@anholt.net


Reviewed-by: Dave Emett <david.emett@broadcom.com>