drm: add func to get max iomem address v2

it will be used to check if the driver needs swiotlb
v2: Don't use inline, instead, move function to drm_memory.c (Michel Daenzer <michel@daenzer.net>)

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180209024410.1469-1-david1.zhou@amd.com