drm/amd/display: dce60_timing_generator: add DCE6 specific functions (v2)

[Why]
DCE6 has CRTC_PREFETCH_EN bit in CRTC_CONTROL register
DCE6 has no CRTC_LEGACY_REQUESTOR_EN bit in CRTC_START_LINE_CONTROL register
DCE6 has no CRTC_CRC_CNTL register

[How]
Modify dce60_timing_generator_enable_advanced_request() function
Add dce60_configure_crc() function and dce60_is_tg_enabled() kept as static
Use dce60_configure_crc() function in dce60_tg_funcs

v2: remove unused variable (Alex)

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>