Screen.dpi is Always Returning 0

Steps to reproduce:

1. Create a Unity project and add a Monobehavior that prints the value of Screen.dpi
2. Build and run the application on an SDL platform (Linux, EmbeddedLinux, or QNX).
3. Check the DPI value of the display.

Actual results: 

DPI is 0.

Expected results: 

Screen.dpi should be set to the correct dpi value of the display.

Observations:

• SDL_GetDisplayDPI returns false and while defaulting dpi values to 0. 
• This bug does not happen on Linux using Wayland.

Reproducible with versions: 

Reproduced on Unity 6000.2.0a8 and 6000.3.0a1

Tested on (OS): 

EmbeddedLinux (using X11 and Wayland)

Linux (using X11)

Not yet tested on QNX.