drm/amdgpu: increase gtt size to 3GB by default v2

v2: address Alex's comment, add AMDGPU_DEFAULT_GTT_SIZE_MB.

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>