drm: Add function to convert rect in 16.16 fixed format to regular format

Much more clear to read one function call than four lines doing this
conversion.

v7:
- function renamed
- calculating width and height before truncate
- inlined

v10:
- renamed parameters from source and destination to src and dst
to match sister functions

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210104205654.238928-1-jose.souza@intel.com