Fixes to CVT3 calcs

Detail:
  Missing break statements (around line 790) would always have selected 50Hz frame rate regardless.
  Removed duplicate call to add_proposed_mode() which generate_mode_using_cvt_rb() already does, resulting in duplicated modes.
Admin:
  CVT3 still untested, but less broken than before.
  Submission for the EDID bounty.
  Tagged as ScrModes-0_55-cvt3fix.

c/edidsupport

@@ -769,10 +769,6 @@ static _kernel_oserror *generate_cvt3_timing(char cvt1, char cvt2, char cvt3, Mo
             }
             IFDEBUG printf("\nAt %x Hz (calculated):\n", freq);
             generate_mode_using_cvt_rb(xres, yres, freq, mode_desc, monitor);
-            if (add_proposed_mode(monitor, mode_desc) == 0)
-            {
-                free(mode_desc);
-            }
         }
         int preferred_freq = freq;
 
@@ -792,18 +788,23 @@ static _kernel_oserror *generate_cvt3_timing(char cvt1, char cvt2, char cvt3, Mo
                     case 0:
                         freq = 60;
                         timing_support = EDID_USE_CVTRB;
+                        break;
                     case 1:
                         freq = 85;
                         timing_support = EDID_USE_CVT;
+                        break;
                     case 2:
                         freq = 75;
                         timing_support = EDID_USE_CVT;
+                        break;
                     case 3:
                         freq = 60;
                         timing_support = EDID_USE_CVT;
+                        break;
                     case 4:
                         freq = 50;
                         timing_support = EDID_USE_CVT;
+                        break;
                 }
 
                 if (freq != preferred_freq)