drm/amd/display: Don't program BLNDGAM_MEM_PWR_FORCE when CM low-power is disabled on DCN30

Before commit 33056a97ae5e ("drm/amd/display: Remove double checks for
`debug.enable_mem_low_power.bits.cm`"), dpp3_program_blnd_lut(NULL)
checked the low-power debug flag before calling
dpp3_power_on_blnd_lut(false).

After commit 33056a97ae5e ("drm/amd/display: Remove double checks for
`debug.enable_mem_low_power.bits.cm`"), dpp3_program_blnd_lut(NULL)
unconditionally calls dpp3_power_on_blnd_lut(false). The BLNDGAM power
helper writes BLNDGAM_MEM_PWR_FORCE when CM low-power is disabled, causing
immediate SRAM power toggles instead of deferring at vupdate. This can
disrupt atomic color/LUT sequencing during transitions between
direct scanout and composition within gamescope's DRM backend on
Steam Deck OLED.

To fix this, leave the BLNDGAM power state unchanged when low-power is
disabled, matching dpp3_power_on_hdr3dlut and dpp3_power_on_shaper.

Fixes: 33056a97ae5e ("drm/amd/display: Remove double checks for `debug.enable_mem_low_power.bits.cm`")
Signed-off-by: Matthew Schwartz <[e-mail]>
Reviewed-by: Harry Wentland <[e-mail]>
Reviewed-by: Mario Limonciello <[e-mail]>
Signed-off-by: Mario Limonciello <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
(cherry picked from commit 13ff4f63fcddfc84ec8632f1443936b00aa26725)
Cc: [e-mail]

drivers/gpu/drm/amd/display/dc/dpp/dcn30/dcn30_dpp.c

···
 			dpp_base->ctx->dc->optimized_required = true;
 			dpp_base->deferred_reg_writes.bits.disable_blnd_lut = true;
 		}
-	} else {
-		REG_SET(CM_MEM_PWR_CTRL, 0,
-				BLNDGAM_MEM_PWR_FORCE, power_on == true ? 0 : 1);
 	}
 }