drm/amdgpu: remove memory training p2c buffer reservation(V2)

IP discovery TMR(occupied the top VRAM with size DISCOVERY_TMR_SIZE)
has been reserved, and the p2c buffer is in the range of this TMR, so
the p2c buffer reservation is unnecessary.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Signed-off-by: Tianci.Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>