drm/amdgpu: Use apt name for FW reserved region

Use the generic term fw_reserved_memory for FW reserve region. This
region may also hold discovery TMR in addition to other reserve
regions. This region size could be larger than discovery tmr size, hence
don't change the discovery tmr size based on this.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>