drm/amdgpu: Fix memory leaks at amdgpu_init() error path

amdgpu driver checks vgacon_text_force() after some initializations
but without cleaning up.  This will result in leaks.

Move the check of vgacon_text_force() to the beginning of
amdgpu_init() for fixing it and also for optimization.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>