2kernel.c: check display request in vb2api_kernel_phase2

In preparation for moving VbTryLoadKernel to depthcharge, move the
VB2_NV_DISPLAY_REQUEST check from vb2api_normal_boot to
vb2api_kernel_phase2.

BUG=b:172339016
BRANCH=none
TEST=make runtests

Signed-off-by: Hsin-Te Yuan <yuanhsinte@google.com>
Change-Id: I3629d8dbbcaba23220152543ba77558c8bbcdc7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3752431
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-by: Hsuan Ting Chen <roccochen@chromium.org>