drm/amd/display: Refactor PSR functionality into dedicated power_psr module

drm/amd/display: Refactor PSR functionality into dedicated power_psr module

[Why]
Extract all PSR (Panel Self Refresh) related functions from power.c
into a new power_psr.c module for better code organization and
maintainability.

[How]
Create new power_psr.c file containing all PSR-related functions
moved from power.c. Remove static qualifier from shared functions
to enable cross-file access:
- psr_context_to_mod_power_psr_context: Convert PSR context to
  module power PSR context
- map_index_from_stream: Map stream to power entity index
- delay_two_frames: Wait for two frame periods

Add function declarations to header. Maintain forward declaration of struct
core_power for type compatibility.

Reviewed-by: Anthony Koo <anthony.koo@amd.com>
Signed-off-by: Lohita Mudimela <lohita.mudimela@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>