poppy: define VbExDisplayGetDimension()

callback to get rows/cols of display

BUG=b:35585623
BRANCH=None
TEST=make sure code compiles

Change-Id: I276975b2f0eecceb66a30ceaa449ab76a440026d
Signed-off-by: Shelley Chen <shchen@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/447837
Reviewed-by: Randall Spangler <rspangler@chromium.org>
2 files changed