tests/amdgpu: Skip multihead MPO tests on single display

[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>

tests/amdgpu/amd_plane.c

@@ -449,9 +449,11 @@ static void test_display_mpo(data_t *data, enum test test, uint32_t format, int
 {
 
 	igt_display_t *display = &data->display;
+	igt_output_t *output;
 	uint32_t regamma_lut_size;
 	lut_t lut;
 	struct fbc fb[4];
+	int valid_outputs = 0;
 	int videos[][2]= {
 		{426, 240},
 		{640, 360},
@@ -464,6 +466,13 @@ static void test_display_mpo(data_t *data, enum test test, uint32_t format, int
 
 	test_init(data);
 
+	/* Skip if there is less valid outputs than the required. */
+	for_each_connected_output(display, output)
+		valid_outputs++;
+
+	igt_skip_on_f(valid_outputs < display_count,
+			"Valid outputs (%d) should be equal or greater than %d\n", valid_outputs, display_count);
+
 	regamma_lut_size = igt_pipe_obj_get_prop(data->pipe[0], IGT_CRTC_GAMMA_LUT_SIZE);
 	igt_assert_lt(0, regamma_lut_size);
 	lut_init(&lut, regamma_lut_size);
@@ -586,9 +595,17 @@ static void test_mpo_swizzle_toggle_multihead(data_t *data)
 {
 	struct amdgpu_bo_metadata meta = {};
 	igt_display_t *display = &data->display;
+	igt_output_t *output;
 	igt_fb_t fb_1280_xr24_tiled, fb_1280_ar24_tiled, fb_1920_xb24_tiled,
 		fb_1920_xb24_linear, fb_1920_xr24_tiled;
 	int w, h;
+	int valid_outputs = 0;
+
+	/* Skip if only one display is connected. */
+	for_each_connected_output(display, output)
+		valid_outputs++;
+
+	igt_skip_on_f(valid_outputs == 1, "Must have more than one output connected\n");
 
 	w = 2400;
 	h = 1350;