drm/i915: Add low level set of routines for programming PM IER/IIR/IMR register set

So far PM IER/IIR/IMR registers were being used only for Turbo related
interrupts. But interrupts coming from GuC also use the same set.
As a precursor to supporting GuC interrupts, added new low level routines
so as to allow sharing the programming of PM IER/IIR/IMR registers between
Turbo & GuC.
Also similar to PM IMR, maintaining a bitmask for PM IER register, to allow
easy sharing of it between Turbo & GuC without involving a rmw operation.

v2:
- For appropriateness & avoid any ambiguity, rename old functions
  enable/disable pm_irq to mask/unmask pm_irq and rename new functions
  enable/disable pm_interrupts to enable/disable pm_irq. (Tvrtko)
- Use u32 in place of uint32_t. (Tvrtko)

v3:
- Rename the fields pm_irq_mask & pm_ier_mask and do some cleanup. (Chris)
- Rebase.

v4: Fix the inadvertent disabling of User interrupt for VECS ring causing
    failure for certain IGTs.

v5: Use dev_priv with HAS_VEBOX macro. (Tvrtko)

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>