drm/agpsupport: Remove assignment in if condition

Move the assignment so that it happens before the if condition.
Merged multiple similar conditionals to a single conditional statement.
This results in syntax which is easier to read.

Found by checkpath.pl

Signed-off-by: Meghana Madhyastha <meghana.madhyastha@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/d30cb59d92c8a98d0665ef6ae2a56b364dc61098.1505376068.git.meghana.madhyastha@gmail.com