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>

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