We already perform the format+mod+rotation test using the small fb, so
we don't actually need to create the big fb until that test has been
performed. We do need to calculate the big fb dimensions though so that
we can calculate the coordinates we're going to use in the test.

$ time kms_big_fb
- real    0m52.902s
+ real    0m52.616s
so not so great.

But if I run each subtest separately it starts to add up:
$ for i in `kms_big_fb --l` ; do kms_big_fb --r $i ; done
- real    5m32.898s
+ real    4m32.164s

The big difference between those two ways of running the test
is at least partially due to the test reusing big fb between
the different rotation subtests for each format+mod combo.
Running each subtest individually can't get that benefit so
avoiding needless big fb creation starts to make a difference.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>