use create_dumb.size as size of buffer

According to `man drm-memory` , "The size field contains the absolute
size in bytes of the buffer. This can normally also be computed with
(height * pitch + width) * bpp / 4". Which suggests that we should use
the .size field to allocate buffer.

Test: th
Test: go to recovery, make sure contents are properly displayed.

BYPASS_INCLUSIVE_LANGUAGE_REASON=commit message referenced "man",
        which is a linux command for lookup manual pages.

Change-Id: I512be6b7d493ef1783f2b7f746e279bc1dfe65f2