radeon: use max_bus_speed to activate gen2 speeds

radeon currently uses a drm function to get the speed capabilities for
the bus, drm_pcie_get_speed_cap_mask. However, this is a non-standard
method of performing this detection and this patch changes it to use
the max_bus_speed attribute.

From: Lucas Kannebley Tavares <lucaskt@linux.vnet.ibm.com>
Signed-off-by: Kleber Sacilotto de Souza <klebers@linux.vnet.ibm.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>