Pixelflinger: Fix issue of pointers being stored in ints

Pixelflinger's code makes assumptions, at certain places,
that pointers can be stored as ints. This patch makes use
of uintptr_t wherever pointers are stored as int or cast
to int.

Change-Id: Ie76f425cbc82ac038a747f77a95bd31774f4a8e8
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>