drm/i915/gt: Update RC6 mask for mtl_drpc

It has been observed sometimes RC6 status register's unused bits are
being set by h/w, without affecting RC6 functionality therefore updating
the mask with used bits accordingly.
As mtl_drpc is debugfs function, removing MISSING_CASE from default case as
it doesn't make sense to panic (panic_on_warn=1) the CI system if register
is reporting unsupported state.

Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Badal Nilawar <badal.nilawar@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230920090620.3255091-1-badal.nilawar@intel.com

drivers/gpu/drm/i915/gt/intel_gt_pm_debugfs.c

@@ -290,7 +290,6 @@ static int mtl_drpc(struct seq_file *m)
 		seq_puts(m, "RC6\n");
 		break;
 	default:
-		MISSING_CASE(REG_FIELD_GET(MTL_CC_MASK, gt_core_status));
 		seq_puts(m, "Unknown\n");
 		break;
 	}

drivers/gpu/drm/i915/gt/intel_gt_regs.h

@@ -26,7 +26,7 @@
 #define   MTL_CAGF_MASK				REG_GENMASK(8, 0)
 #define   MTL_CC0				0x0
 #define   MTL_CC6				0x3
-#define   MTL_CC_MASK				REG_GENMASK(12, 9)
+#define   MTL_CC_MASK				REG_GENMASK(10, 9)
 
 /* RPM unit config (Gen8+) */
 #define RPM_CONFIG0				_MMIO(0xd00)