drm/nouveau/gsp: fix vblank interrupts on GB20x

The GSP path programs per-head timing (vblank) interrupts the same way on
every generation. NVD5.0 (GB20x) reworked the FE interrupt frontend
around four message-based kernel vectors (high latency, low latency, PMU,
and GSP) and moved RM head-timing interrupts to the dedicated low-latency
vector:

 - The enable is NV_PDISP_FE_RM_INTR_EN1_HEAD_TIMING, 0x611ef0 +
   head*4 (570.144 kernel_head_0501.c, renamed kernel_head_0502.c from
   575.51.02 on, and v05_01 dev_disp.h).

 - The vector is reported as a separate interrupt table entry,
   MC_ENGINE_IDX_DISP_LOW (intr_gb202.c, intrCacheDispIntrVectors).

 - The vector must be re-armed through NV_PDISP_FE_INTR_RETRIGGER(1)
   at 0x611f34 after servicing (kdispServiceInterrupt ->
   kdispIntrRetrigger_v05_01).

The event latch (0x611800), per-head status (0x611c00), and dispatch
summary (0x611ec0) the interrupt handler uses are unchanged on GB20x
(kheadReadPendingVblank_v03_00 and kheadResetPendingLastData_v03_00
remain for DISPv0502+).

On GB20x the old code enables head timing onto the legacy vector, leaves
its handler there, and never re-arms the message-based vectors. Page
flips still complete (nv50 sends those events from the commit path), so
the desktop looks fine while DRM vblank waits and vblank sequence queries
are affected.

Supply GB20x vblank enables and an interrupt handler that re-arms the
vector after servicing through gb202_gsp_disp, translate the low-latency
interrupt table entry as a second NVKM_ENGINE_DISP instance, and add a
gsp.intr_low_latency flag so r535_disp_oneinit() attaches the handler to
that instance. GB20x was the last cross-file user of the TU1xx vblank
enables, so make those static and drop their head.h prototypes.

Fixes: 6cc6e08d4542 ("drm/nouveau/kms: add support for GB20x")
Cc: stable@vger.kernel.org
Signed-off-by: Mohamed Ahmed <mohamedahmedegypt2001@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20260825001408.14219-9-mohamedahmedegypt2001@gmail.com

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