drm/i915/uc: Rationalise delimiters in filename macros

The way delimiters (underscores and dots) were added to the UC
filenames was different for different types of delimiter. Rationalise
them to all be done the same way - implicitly in the concatenation
macro rather than explicitly in the file name prefix.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221129232031.3401386-2-John.C.Harrison@Intel.com