drm/i915: get runtime PM at i915_reg_read_ioctl

To avoid WARNs when we call it.

Testcase: igt/pm_pc8/reg-read-ioctl
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75693


Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>