selftests/powerpc: Skip earlier in alignment_handler test

Currently the alignment_handler test prints "Can't open /dev/fb0"
about 80 times per run, which is a little annoying.

Refactor it to check earlier if it can open /dev/fb0 and skip if not,
this results in each test printing something like:

  test: test_alignment_handler_vsx_206
  tags: git_version:v4.18-rc3-134-gfb21a48904aa
  [SKIP] Test skipped on line 291
  skip: test_alignment_handler_vsx_206

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>