drm/i915: sanitize RPS resetting during GPU reset

Atm, we don't disable RPS interrupts and related work items before
resetting the GPU. This may interfere with the following GPU
initialization and cause RPS interrupts to show up in PM_IIR too early
before calling gen6_enable_rps_interrupts() (triggering a WARN there).

Solve this by disabling RPS interrupts and flushing any related work
items before resetting the GPU.

v2:
- split out the common parts of the gt suspend and the new gt reset
  functions (Paulo)
v3:
- remove the check for UMS, it's a NOP nowadays (Daniel)

Reported-by: He, Shuang <shuang.he@intel.com>
Testcase: igt/gem_reset_stats/ban-render
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86644
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>