Increasing max pipe count to 6 to support AMD GPU's.

Since some tests' behavior depends on this value, small changes are made
to remove this dependency:

* kms_ccs: Early abort if wanted_pipe is out-of-bounds.
* kms_concurrent: Check if pipe is within bounds first.
* kms_pipe_color: Prevent skipping of subsequent tests by placing
    generated tests in a 'igt_subtest_group'.
* kms_plane: Move pipe and plane index checking to subtest group level.

v2: Change invalid pipe check on kmstest_pipe_name() to use
I915_MAX_PIPE

v3: Change tabs to spaces in 'enum pipe'

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Tested-by: Tomi Sarvela <tomi.p.sarvela@intel.com>