Fix armhf configure breakage due to missing libdrm_intel package

Fix armhf configure breakage due to missing libdrm_intel package

The buildbots discovered that recent changes break on Ubuntu 15.04's
armhf images:

  configure:16137: checking for SIMPLE_DMABUF_CLIENT
  configure:16144: $PKG_CONFIG --exists --print-errors "wayland-client libdrm libdrm_intel"
  Package libdrm_intel was not found in the pkg-config search path.
  ...
  configure:16194: error: Package requirements (wayland-client libdrm libdrm_intel) were not met:

  No package 'libdrm_intel' found

This patch was provided by Daniel Stone.  I've not tested it other than
verifying it does not cause build problems on x86_64.

Acked-by: Bryce Harrington <bryce@osg.samsung.com>