drm/i915: Simplify intel_gpu_reset

Replaced ever growing switch for gen version with chained conditionals.
Futre gen's only need to add a new one if they require something different.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Robert Beckett <robert.beckett@intel.com>
[danvet: Picked from internal tree and white-wash commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>