I don't remember where this is explained in the u-boot/kernel code.
But the BeagleBone Black's AM335x just doesn't have enough memory
bandwidth to fully support "large" screen sizes..
This is why 1080p maxes out at 30 fps, or 24 fps when audio is also involved..
The 'e' is documented here:
"DRM drivers also add options to enable or disable outputs:
'e' will force the display to be enabled, i.e. it will override the detection
if a display is connected. 'D' will force the display to be enabled and use
digital output. This is useful for outputs that have both analog and digital
signals (e.g. HDMI and DVI-I). For other outputs it behaves like 'e'. If 'd'
is specified the output is disabled."...
Regards,