drm/amdgpu: reduce ip discovery data reading size

Only read first 4K data instead of whole TMR block,
so we can reduce the time in full access mode.

Signed-off-by: Wenhui Sheng <Wenhui.Sheng@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>