soc/intel/{adl, mtl}: Avoid redundant display init by joining to MBUS

This patch ensures that the IGD joins the MBUS when the firmware splash
screen feature is enabled (aka BMP_LOGO config is enabled).

For ChromeOS platform, it prevents the i915 driver from reinitializing
the display, which can save up to 75ms-80ms of boot time and eliminate
a brief period of blank screen between the firmware splash screen and
the OS login prompt.

BUG=b:284799726
TEST=Able to build and boot google/rex.

Change-Id: I36af167afa902053a987602d494a8830ad9b1b1a
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78387
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <ericllai@google.com>
Reviewed-by: Nick Vaccaro <nvaccaro@google.com>

src/soc/intel/alderlake/Kconfig

@@ -82,6 +82,7 @@ config SOC_INTEL_ALDERLAKE
 	select SOC_INTEL_COMMON_RESET
 	select SOC_INTEL_CSE_SEND_EOP_LATE if !BOARD_GOOGLE_BRYA_COMMON
 	select SOC_INTEL_CSE_SET_EOP
+	select SOC_INTEL_GFX_MBUS_JOIN if MAINBOARD_HAS_CHROMEOS && BMP_LOGO
 	select SOC_INTEL_MEM_MAPPED_PM_CONFIGURATION
 	select HAVE_INTEL_COMPLIANCE_TEST_MODE
 	select SSE2

src/soc/intel/meteorlake/Kconfig

@@ -89,6 +89,7 @@ config SOC_INTEL_METEORLAKE
 	select SOC_INTEL_CSE_SEND_EOP_LATE if !MAINBOARD_HAS_CHROMEOS
 	select SOC_INTEL_CSE_SET_EOP
 	select SOC_INTEL_IOE_DIE_SUPPORT
+	select SOC_INTEL_GFX_MBUS_JOIN if MAINBOARD_HAS_CHROMEOS && BMP_LOGO
 	select SOC_INTEL_GFX_NON_PREFETCHABLE_MMIO
 	select SOC_INTEL_MEM_MAPPED_PM_CONFIGURATION
 	select SSE2