[cogl-bitmap] Fix minor copy and paste error in _cogl_bitmap_fallback_premult
[cogl-bitmap] Fix minor copy and paste error in _cogl_bitmap_fallback_premult

The returned bitmap format should include the COGL_PREMULT_BIT flag not
have it explicitly removed as for _cogl_bitmap_fallback_unpremult.

collects/clutter,heysion/clutter-clone,ebassi/clutter,jigpu/clutter,jigpu/clutter,djdeath/clutter,tardyp/clutter,Distrotech/clutter,dlespiau/clutter,collects/clutter,kerrickstaley/clutter-vala,spatulasnout/clutter,djdeath/clutter-multithreaded,djdeath/clutter,dlespiau/clutter,ebassi/clutter,djdeath/clutter,heysion/clutter-clone,ebassi/clutter,ebassi/clutter,Distrotech/clutter,Distrotech/clutter,collects/clutter,spatulasnout/clutter,heysion/clutter-clone,nobled/clutter,rkudiyarov/clutter_osx,djdeath/clutter-multithreaded,GNOME/clutter,ebassi/clutter,djdeath/clutter-android,dlespiau/clutter,spatulasnout/clutter,jigpu/clutter,GNOME/clutter,djdeath/clutter-multithreaded,GNOME/clutter,djdeath/clutter,Distrotech/clutter,djdeath/clutter,jigpu/clutter,heysion/clutter-clone,kerrickstaley/clutter-vala,djdeath/clutter-android,rkudiyarov/clutter_osx,kerrickstaley/clutter-vala,djdeath/clutter,spatulasnout/clutter,rkudiyarov/clutter_osx,collects/clutter,GNOME/clutter,tardyp/clutter,djdeath/clutter-android,kerrickstaley/clutter-vala,djdeath/clutter-android,nobled/clutter,rkudiyarov/clutter_osx,tardyp/clutter,GNOME/clutter,Distrotech/clutter,djdeath/clutter-android,Distrotech/clutter,spatulasnout/clutter,jigpu/clutter,nobled/clutter,collects/clutter,heysion/clutter-clone,jigpu/clutter,djdeath/clutter-multithreaded