drm/radeon: Inline drm_get_pci_dev

It's the last user, and more importantly, it's the last non-legacy
user of anything in drm_pci.c.

The only tricky bit is the agp initialization. But a close look shows
that radeon does not use the drm_agp midlayer (the main use of that is
drm_bufs for legacy drivers), and instead could use the agp subsystem
directly (like nouveau does already). Hence we can just pull this in
too.

A further step would be to entirely drop the use of drm_device->agp,
but feels like too much churn just for this patch.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "David (ChunMing) Zhou" <David1.Zhou@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org