Fix #33809: Wrong exposure affect after alpha premul changes

Exposure shouldn't affect on alpha channel, only RGB shall be
affected by exposure.

Was a regression since recent alpha premul pipeline changes.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53667 954f8c5b-7b00-dc11-b283-0030488c597c