drm/i915/de: Implement register polling in the display code

The plan is to move all the mmio stuff into the display code itself.
As a first step implement the register polling in intel_de.c.

Currently i915 and xe implement this stuff in slightly different
ways, so there are some functional changes here. Try to go for a
reasonable middle ground between the i915 and xe implementations:
- the exponential backoff limit is the simpler approach taken
by i915 (== just clamp the max sleep duration to 1 ms)
- the fast vs. slow timeout handling is similar to i915 where
we first try the fast timeout and then again the slow timeout
if the condition still isn't satisfied. xe just adds up the
timeouts together, which is a bit weird.
- the atomic wait variant uses udelay() like xe, whereas i915
has no udelay()s in its atomic loop. As a compromise go for a
fixed 1 usec delay for short waits, instead of the somewhat
peculiar xe behaviour where it effectively just does one
iteration of the loop.
- keep the "use udelay() for < 10 usec waits" logic (which
more or less mirrors fsleep()), but include an explicit
might_sleep() even for these short waits when called from
a non-atomic intel_de_wait*() function. This should prevent
people from calling the non-atomic functions from the wrong
place.

Eventually we may want to switch over to poll_timeout*(),
but that lacks the exponential backoff, so a bit too
radical to change in one go.

v2: Initialize ret in intel_de_wait_for_register() to avoid a
warning from the compiler. This is actually a false positive
since we always have fast_timeout_us!=0 when slow_timeout_us!=0,
but the compiler can't see that

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260323094304.8171-1-ville.syrjala@linux.intel.com