gpu: amd: replace open-coded offsetof() with builtin

The two AMD drivers have their own custom offsetof() implementation
that now triggers a warning with recent versions of clang:

drivers/gpu/drm/radeon/radeon_atombios.c:133:14: error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction]

Change all the instances to use the normal offsetof() provided
by the kernel that does not have this problem.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>