Use strcpy for RandR output property names

Instead of strncpy with the string length. Avoids new warnings with GCC
8:

../../src/drmmode_display.c: In function ‘drmmode_output_create_resources’:
../../src/drmmode_display.c:2240:2: warning: ‘strncpy’ output truncated before terminating nul copying 8 bytes from a string of the same length [-Wstringop-truncation]
  strncpy(tearfree_prop->name, "TearFree", 8);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../src/drmmode_display.c:2244:2: warning: ‘strncpy’ output truncated before terminating nul copying 3 bytes from a string of the same length [-Wstringop-truncation]
  strncpy(tearfree_prop->enums[0].name, "off", 3);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../src/drmmode_display.c:2245:2: warning: ‘strncpy’ output truncated before terminating nul copying 2 bytes from a string of the same length [-Wstringop-truncation]
  strncpy(tearfree_prop->enums[1].name, "on", 2);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../src/drmmode_display.c:2247:2: warning: ‘strncpy’ output truncated before terminating nul copying 4 bytes from a string of the same length [-Wstringop-truncation]
  strncpy(tearfree_prop->enums[2].name, "auto", 4);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

(Ported from amdgpu commit f3b2ed37d683f8616a0a31ff63133ddb8fe1a4a3)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>