drm/i915: Use atomic waits for short non-atomic ones

usleep_range is not recommended for waits shorten than 10us.

Make the wait_for_us use the atomic variant for such waits.

To do so we need to reimplement the _wait_for_atomic macro to
be safe with regards to preemption and interrupts.

v2: Reimplement _wait_for_atomic to be irq and preemption safe.
    (Chris Wilson and Imre Deak)

v3: Fixed in_atomic check due rebase error.
v4: Build bug on non-constant timeouts.
v5: Compile away cpu migration code in atomic paths.

Signed-off-by: Tvrtko Ursulin <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Imre Deak <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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