drm/i915: s/GEN5/GEN3/

The GEN5_IRQ_RESET/INIT macros are perfectly suitable even for
gen3/4 hardware as those have 32 bit interrupt registers. Let's
rename the macros to reflect that fact.

Gen2 on the other hand has 16 bit interrupt registers so these
macros aren't really appropriate there.

v2: Fix patch subject (Maarten)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-4-ville.syrjala@linux.intel.com