drm/i915/dp: Remove intel_ prefix from hw signal_levels functions

intel_ is for generic code bxt_ and friends for platform specific
functions. Remove the intel_ prefix to be consistent with our naming.

Random OCD bikeshed I've spotted while merging bxt patches.

v2: Oops, git add fail.

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>