drm/amd/display: Refactor DPG test pattern logic for ODM cases

[Why]
Current DPG test pattern logic does not account for ODM configuration
changes after test pattern has already been programmed. For example, if
ODM2:1 is enabled after test pattern is already being output, the second
pipe is not programmed to output test pattern, causing half the screen
to be black.

[How]
Move DPG test pattern parameter calculations into separate function.
Whenever ODM pipe configuration changes, re-calculate DPG test pattern
parameters and program DPG if test pattern is currently enabled.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>