drm/i915: Use genX_ prefix for gt irq enable/disable functions

Traditionally we use genX_ for GT/render stuff and the codenames for
display stuff. But the gt and pm interrupt handling functions on
gen5/6+ stuck out as exceptions, so convert them.

Looking at the diff this nicely realigns our ducks since almost all
the callers are already platform-specific functions following the
genX_ pattern.

Spotted while reviewing some internal rps patches.

No function change in this patch.

Acked-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>