drm/radeon: remove the repeated declaration

Function 'r300_mc_wait_for_idle' and 'r600_mc_wait_for_idle'
are declared twice, remove the repeated declaration.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>