drm/i915/display: Avoid stale PIPE_SCANLINE values after crtc_enable

drm/i915/display: Avoid stale PIPE_SCANLINE values after crtc_enable

When a CRTC is moved to a different transcoder (e.g. on DP-MST stream
allocation), PIPE_SCANLINE and PIPE_FRMCNT can return values latched
from the previous transcoder/mode for up to one vblank period after
the new pipe is enabled. The vblank evasion code in
intel_pipe_update_start()/end() then samples a stale or boundary
scanline and the frame counter ticks during the critical section,
producing diagnostic errors of the form:

  [243.348405] xe 0000:00:02.0: [drm] *ERROR* Atomic update failure on
pipe B (start=300 end=301) time 61 us, min 2128, max 2161, scanline
start 1200, end 2165
  [248.536260] xe 0000:00:02.0: [drm] *ERROR* Atomic update failure on
pipe B (start=561 end=562) time 61 us, min 2128, max 2161, scanline
start 2162, end 2167

Here "scanline start 1200" is the vblank_start of a previously
programmed mode on a different transcoder, while "2162" is the current
mode's vblank_start sampled before any real frame has been emitted.
Both indicate a stale read rather than a real evasion miss.

Wait for one vblank after crtc_enable() to give the new transcoder a
chance to start producing live PIPE_SCANLINE/FRMCNT values before any
subsequent atomic commit enters the vblank evasion section. This adds
at most one frame of latency on modeset, which is invisible to users.

Reproduced with igt@kms_rotation_crc@sprite-rotation-180 on a DP-MST
sink; with this patch the failures no longer occur.

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Pranay Samala <pranay.samala@intel.com>
Link: https://patch.msgid.link/20260429042650.3335718-1-suraj.kandpal@intel.com