[PREEMPT_RT] bogus lockdep assert from i915 on v5.2-rt1

[Clark Williams]

 Thomas,

When looking at a problem on v5.2-rt1, I turned on lockdep and started getting warnings
from lockdep_assert_irqs_disabled() in the i915 driver. They're making these calls inside
a spin_lock_irqsave/spin_lock_irqrestore block, which of course doesn't fiddle with IRQs
when PREEMPT_RT is configured. The attached patch places the three calls inside
if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) blocks, so should avoid the bogus warning on RT.

Clark

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