drm/i915/dp: Refactor intel_dp_in_hdr_mode() for broader reuse

drm/i915/dp: Refactor intel_dp_in_hdr_mode() for broader reuse

The intel_dp_in_hdr_mode() helper was previously defined in
intel_dp_aux_backlight.c but is generally useful beyond that
context. Move the function to intel_dp.c and declare it in
intel_dp.h to make it accessible to other DP-related code
paths that need to check HDR metadata state.

This is a pure refactor with no functional change and
prepares for a follow-up patch that uses this helper.

Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/20250730055523.2214966-2-chaitanya.kumar.borah@intel.com