minui: fix screen update issue

yres_virtual value is set incorrectly, causing serveral images to be skipped.
Change this value according to the number of buffers to fix this issue.

from: codeaurora.org
minui: Display the battery charging image correctly
commit: 581a4dead6b96579a13ff22e2454c1f329731679

Change-Id: I10f5d1c6cc37705f0287c7dd517082de2e11d264
Signed-off-by: Iliyan Malchev <malchev@google.com>