drm/radeon: Fix style issues in radeon _encoders.c & _gart.c

Conform to Linux kernel coding style.

Fixes the following & other checks in radeon_encoders.c & radeon_gart.c:

WARNING: Missing a blank line after declarations
WARNING: Block comments use * on subsequent lines
WARNING: Prefer 'unsigned int' to bare use of 'unsigned'
WARNING: braces {} are not necessary for single statement blocks

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: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>