drm/amdgpu: remove VRAM size reduction v2

Remove some outdated comments and all code which tries to reduce the VRAM size
mapped into the MC.

This is superfluous and misleading since we never actually program the size.

v2: handle gmc_v6_0.c as well

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