drm/vc4: perfmon: Fix variable dereferenced before check

Commit 30f8c74c ("drm/vc4: Warn if some v3d code is run on BCM2711")
introduced a check in vc4_perfmon_get() that dereferences a pointer before
we checked whether that pointer is valid or not.

Let's rework that function a bit to do things in the proper order.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 30f8c74c ("drm/vc4: Warn if some v3d code is run on BCM2711")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220622080243.22119-1-maxime@cerno.tech