tests/i915/kms_big_fb: optimize async flip tests

on async flip tests there's need to create large solid color
framebuffer. Here is optimized creating this framebuffer
with help of rendercopy.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>