drm/i915: move RPS PM_IER enabling to gen6_enable_rps_interrupts

Paulo noticed that we don't enable RPS interrupts via PM_IER in
gen6_enable_rps_interrupts(). This wasn't a problem so far, since the
only place we disabled RPS interrupts was during system/runtime suspend
and after that we reenable all interrupts in the IRQ pre/postinstall
hooks.

In the next patch we'll disable/reenable RPS interrupts during GPU reset
too, but not call IRQ uninstall, pre/postinstall hooks, so there the
above wouldn't work. The logical place for programming PM_IER is
gen6_enable_rps_interrupts() and this also makes the function more
symmetric with gen6_disable_rps_interrupts(), so move the programming
there from the postinstall hooks.

Note that these changes don't affect the ILK RPS interrupt code, which
could be sanitized in a similar way. But that can be done as a
follow-up.

Credits-to: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>