lib: add igt_debugfs_read()

A helpful function for when you want to read a whole debugfs file to a
string and don't want to worry about opening and closing file
descriptors and asserting buffer sizes.

We've been using this already for kms_frontbuffer_tracking and
kms_fbcon_fbt, so the only test with new code here is kms_fbc_crc.

Also notice that for kms_fbc_crc we had to increase the buffer size
since the file can sometimes be bigger than 64 bytes - depending on
the reason why FBC is disabled.

Of course, there are probably many other programs we can patch, but
I'm not doing this now.

v2: Add the macro to wrap sizeof() (Daniel).
v3: Add documentation for the macro too (Daniel).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>

tests/kms_fbcon_fbt.c

@@ -86,28 +86,11 @@ static void teardown_drm(struct drm_info *drm)
 	igt_assert(close(drm->fd) == 0);
 }
 
-static void debugfs_read(const char *filename, char *buf, int buf_size)
-{
-	FILE *file;
-	size_t n_read;
-
-	file = igt_debugfs_fopen(filename, "r");
-	igt_assert(file);
-
-	n_read = fread(buf, 1, buf_size - 1, file);
-	igt_assert(n_read > 0);
-	igt_assert(feof(file));
-
-	buf[n_read] = '\0';
-
-	igt_assert(fclose(file) == 0);
-}
-
 static bool fbc_supported_on_chipset(void)
 {
 	char buf[128];
 
-	debugfs_read("i915_fbc_status", buf, ARRAY_SIZE(buf));
+	igt_debugfs_read("i915_fbc_status", buf);
 	return !strstr(buf, "FBC unsupported on this chipset\n");
 }
 
@@ -120,7 +103,7 @@ static bool fbc_is_enabled(void)
 {
 	char buf[128];
 
-	debugfs_read("i915_fbc_status", buf, ARRAY_SIZE(buf));
+	igt_debugfs_read("i915_fbc_status", buf);
 	return strstr(buf, "FBC enabled\n");
 }
 
@@ -172,7 +155,7 @@ static bool psr_supported_on_chipset(void)
 {
 	char buf[256];
 
-	debugfs_read("i915_edp_psr_status", buf, ARRAY_SIZE(buf));
+	igt_debugfs_read("i915_edp_psr_status", buf);
 	return strstr(buf, "Sink_Support: yes\n");
 }
 
@@ -185,7 +168,7 @@ static bool psr_is_enabled(void)
 {
 	char buf[256];
 
-	debugfs_read("i915_edp_psr_status", buf, ARRAY_SIZE(buf));
+	igt_debugfs_read("i915_edp_psr_status", buf);
 	return strstr(buf, "\nActive: yes\n");
 }