drm/i915/fdi: split out FDI regs to a separate file

Clean up i915_reg.h by splitting out FDI regs to
display/intel_fdi_regs.h.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/bba37e46d767e2193d49d1d2e289040c6bf8229b.1678973282.git.jani.nikula@intel.com