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The 72x40 OLED requires selecting the internal IREF, as opposed to the
default external IREF.  This is an undocumented feature in the SSD1306
datasheet, but is present in the SSD1315 datasheet.  It's possible the
72x40 OLED is actually using the newer SSD1315 controller.  Sending the
IREF select command to SSD1306 displays has no effect on them, so it's
added to the init_display() instead of wrapping in an "if width = 72".

Also tested on a 128x64 OLED using the SSD1315 controller (smaller ribbon
cable) and the proposed change has no effect on the display, as the module
comes with the correct current limiting resistor.  Internal and external
IREF work the same.

Fixes issue #7281.