UPSTREAM: drm/i915: Introduce intel_scanlines_to_usecs()

Introduce intel_scanlines_to_usecs() as a counterpart to
intel_usecs_to_scanlines().

We'll have some use for this in DSB code as we want to do
relative scanline waits to evade the delayed vblank, but
unfortunately DSB can't do relative scanline waits (only
absolute). So we'll instead convert the relative scanline
count to usec and do a relative usec wait instead (which the
DSB knows how to do).

Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930170415.23841-9-ville.syrjala@linux.intel.com


(cherry picked from commit d4f9a053)

Bug: 406266020
Bug: 387910155
Test: DUT booted to UI

Change-Id: I0661f0f94f2df48b6e23b2b1f99760e132cfbc8b
Signed-off-by: George D Sworo <george.d.sworo@intel.corp-partner.google.com>
Signed-off-by: Ryan Neph <ryanneph@google.com>