pixelflinger: Use pointer arithmetic to determine cache flush parameters

CodeCache casts base address to long and then adds size (of type
ssize_t) to get end address. This can cause sign-extension problems.
This patch instead uses simple pointer arithmetic.

Change-Id: Ib71d515a6fd6a7f4762cf974d6cf4eba9a601fa8
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>