drm/amdgpu: correct discovery_tmr_size init val

The legacy way to initialize discovery_tmr_size
is using DISCOVERY_TMR_SIZE, while after we reduce
DISCOVERY_TMR_SIZE from 64KB to 4KB, variable
discovery_tmr_size is also reduced to 4KB, this is not
correct, it still should be 64KB, discovery_tmr_size
will be used to calculate ip_discovery reserved mem's
start address and size.

Using DISCOVERY_TMR_OFFSET to init discovery_tmr_size instead.

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>