drm/i915: remove useless runtime PM get calls

We already call intel_display_power_get, which will get a power
domain, and every power domain should get a runtime PM reference,
which will wake up the machine.

v2: - Also touch intel_crt_detect() (Ville).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Fixup commit message as spotted by Ville.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>