tests/kms_plane_multiple: Describe the test

In this test all the subtest are doing the same exact thing but with
different tiling / on a different pipe. Tiling/pipe is in the test name
so no need to elaborate on them.

We can have just one igt_describe() on top of the group to describe all
the subtests in one go, no copy-and-paste necessary.

The description is short and explains the spirit of the test (verifying
atomicity of plane updates) without delving into the implementation
details (holes, colors, etc.)

Other changes:
 * The function used for grabbing reference CRC is now under a more
   descriptive name.
 * Comment explaining test implementation in more detail is moved to the
   top of the function actually implementing those steps.
 * Minor formatting touch ups.

Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Acked-by: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>