drm/amd/amdgpu: remove vram_page_split kernel option (v3)

This option is no longer needed.  The default code paths
are now the only option.

v2: Add HPAGE support and a default for non contiguous maps
v3: Misread 512 pages as MiB ...

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>