drm/i915: get runtime PM while trying to detect CRT

Otherwise we'll read registers that return 0xffffffff, trigger some
WARNs, think CRT is actually connected (because certain bits are 1),
and fail the drm-resources-equal testcase!

Tested on a SNB machine with runtime PM support (which is not upstream
yet, but is already on my public tree at freedesktop.org, and will
hopefully eventually become upstream).

Testcase: igt/pm_pc8/drm-resources-equal
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>