drm/amdgpu: add new gttsize module parameter v2

This allows setting the gtt size independent of the gart size.

v2: fix copy and paste typo

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>