drm/radeon: Avoid externs & do not initialize globals to 0 in radeon_drv.c

Fixes the following:

 - WARNING: externs should be avoided in .c files
 - ERROR: do not initialise globals to 0
 - WARNING: Missing a blank line after declarations

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>