drm/nouveau/kms/nv50-: Program notifier offset before requesting disp caps

drm/nouveau/kms/nv50-: Program notifier offset before requesting disp caps

Not entirely sure why this never came up when I originally tested this
(maybe some BIOSes already have this setup?) but the ->caps_init vfunc
appears to cause the display engine to throw an exception on driver
init, at least on my ThinkPad P72:

nouveau 0000:01:00.0: disp: chid 0 mthd 008c data 00000000 0000508c 0000102b

This is magic nvidia speak for "You need to have the DMA notifier offset
programmed before you can call NV507D_GET_CAPABILITIES." So, let's fix
this by doing that, and also perform an update afterwards to prevent
racing with the GPU when reading capabilities.

v2:
* Don't just program the DMA notifier offset, make sure to actually
  perform an update
v3:
* Don't call UPDATE()
* Actually read the correct notifier fields, as apparently the
  CAPABILITIES_DONE field lives in a different location than the main
  NV_DISP_CORE_NOTIFIER_1 field. As well, 907d+ use a different
  CAPABILITIES_DONE field then pre-907d cards.
v4:
* Don't forget to check the return value of core507d_read_caps()
v5:
* Get rid of NV50_DISP_CAPS_NTFY[14], use NV50_DISP_CORE_NTFY
* Disable notifier after calling GetCapabilities()

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 4a2cb4181b07 ("drm/nouveau/kms/nv50-: Probe SOR and PIOR caps for DP interlacing support")
Cc: <stable@vger.kernel.org> # v5.8+
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>