drm/i915/selftests: fix smatch warning in igt_check_blocks

The block here can't be NULL, especially since we already dereferenced
it earlier, so remove the redundant check.

igt_check_blocks() warn: variable dereferenced before check 'block' (see line 126)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702104642.1189978-1-matthew.auld@intel.com