drm/i915: move i915_reset_count()/i915_reset_engine_count() out of i915_drv.h

It doesn't help much, as i915_drv.h includes i915_gpu_error.h, but it's
a step in the right direction.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7af2f698a320c1efb0563f56a432c6d122d40b94.1644507885.git.jani.nikula@intel.com