drm/amd/display: replace fast_validate with enum dc_validate_mode

[Why]
The boolean fast_validate is used as an
input parameter in multiple functions. To
support more scenarios, we are
replacing it with enum dc_validate_mode.

[How]
The enum dc_validate_mode introduces three
possible values:
1) DC_VALIDATE_MODE_AND_PROGRAMMING:
   Apply the mode to hardware
2) DC_VALIDATE_MODE_ONLY:
   Check whether the mode can be supported
3) DC_VALIDATE_MODE_AND_STATE_INDEX:
   Check if the mode can be supported, and
   determine the optimal voltage level
   needed to support it.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Yan Li <yan.li@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>