drm/amd/display: Allow backlight to go below `AMDGPU_DM_DEFAULT_MIN_BACKLIGHT`

The issue with panel power savings compatibility below
`AMDGPU_DM_DEFAULT_MIN_BACKLIGHT` happens at
`AMDGPU_DM_DEFAULT_MIN_BACKLIGHT` as well.

That issue will be fixed separately, so don't prevent the backlight
brightness from going that low.

Cc: Harry Wentland <[e-mail]>
Cc: Thomas Weißschuh <[e-mail]>
Link: https://lore.kernel.org/amd-gfx/[e-mail]/T/#m400dee4e2fc61fe9470334d20a7c8c89c9aef44f
Reviewed-by: Harry Wentland <[e-mail]>
Signed-off-by: Mario Limonciello <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

···
 		int spread = caps.max_input_signal - caps.min_input_signal;
 
 		if (caps.max_input_signal > AMDGPU_DM_DEFAULT_MAX_BACKLIGHT ||
-		    caps.min_input_signal < AMDGPU_DM_DEFAULT_MIN_BACKLIGHT ||
+		    caps.min_input_signal < 0 ||
 		    spread > AMDGPU_DM_DEFAULT_MAX_BACKLIGHT ||
 		    spread < AMDGPU_DM_MIN_SPREAD) {
 			DRM_DEBUG_KMS("DM: Invalid backlight caps: min=%d, max=%d\n",