drm/i915: propagate the error code from runtime PM callbacks

Atm, none of the RPM callbacks can fail, but the next patch adding
RPM support for VLV changes this, so prepare for it.

In case one of these callbacks return error RPM will get permanently
disabled until the error is explicitly cleared. In the future we could
add support for re-enabling it, for example after resetting the HW, but
for now - hopefully - we can live with the simpler solution.

v2:
- propagate the error from the resume callbacks too (Paulo)
v3:
- fix rebase fail typo around IS_GEN6() check in intel_runtime_suspend()

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>