[mlir] Fix case in MLIRGPUtoNVVMTransforms

Summary:
This changes the casing of MLIRGPUtoNVVMTransforms to be consistent
with other transform libraries.

Reviewers: stephenneuendorffer, herhut

Reviewed By: herhut

Subscribers: jholewinski, mgorny, mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, csigg, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, Joonsoo, grosul1, Kayjukh, jurahul, msifontes

Tags: #mlir

Differential Revision: https://reviews.llvm.org/D82842