drm/i915: Use drm_i915_private directly from drv_get_drvdata()

As we store a pointer to i915 in the drvdata field (as the pointer is both
an alias to the drm_device and drm_i915_private), we can use the stored
pointer directly as the i915 device.

v2: Store and use i915 inside drv_get_drvdata()
v3: Only expect i915 inside drv_get_drvdata() so drop the assumed
i915/drm equivalence.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190806074219.11043-1-chris@chris-wilson.co.uk