[Why]
If only one output is connected, all the igt@amdgpu/amd_plane "-multihead"
subtests will result in CRASH, such as can be seen in:

Starting subtest: mpo-scale-nv12-multihead
Received signal SIGSEGV.
Stack trace:
 #0 [fatal_sig_handler+0x163]
 #1 [__sigaction+0x50]
 #2 [force_output_mode+0xa]
 #3 [test_display_mpo+0x20b]
 #4 [__igt_unique____real_main725+0x106d]
 #5 [main+0x23]
 #6 [__libc_start_call_main+0x80]
 #7 [__libc_start_main+0x89]
 #8 [_start+0x25]
Subtest mpo-scale-nv12-multihead: CRASH (0.009s)

In single-display systems, the "-multihead" subtests should SKIP.

[How]
Count the number of connected outputs on the system and compare it to the
ideal number depending on the subtest, using igt_skip_on to assure that
multiheaded subtests will SKIP on single display systems.

Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Reviewed-by: Alex Hung <alex.hung@amd.com>