tests/kms_vrr: Move fb0 and fb1 to an array

Consolidate the 2 fb variables into an array. No functional changes.

v2:
- Clarified commit msg (Bhanu)
v3: (Bhanu)
- Rebase

Cc: Mark Yacoub <markyacoub@chromium.org>
Reviewed-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>